So your contract is up: What if you just kept your old phone?

Readers have asked us about whether it's possible to just keep an old phone and save money, instead of upgrading when a contract ends. In this helpful article, David Gewirtz walks you through the risks and benefits of doing so.

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"The Times They Are a-Changin'." So sang Bob Dylan, the poet-laureate of rock 'n roll. They are. But the more things change, the more they stay the same.

Take, for example, phone contracts. Up until recently, most of us bought our phones along with phone service, and most of the cost of our expensive smartphones was buried deep inside the carrier "subsidy" that came attached with chains to our freedom. We were locked into a single carrier (at whatever unacceptable rate or lack of quality service) until the contract ended.

As of this summer, all major carriers (with the exception of AT&T) have done away with carrier subsidies. You can bring your own phone. Of course, that means you're spending $400-800+ outright on an unlocked phone, and probably paying usurious interest to Visa or Mastercard.

The alternative is that some carriers are selling the phones to you on installment plans (often with no interest), for $20-35/mo, effectively loaning you the amount to pay off the phone. The gotcha -- you have to have a phone plan with them, and if you cancel the phone plan, your remaining balance on the phone becomes a pumpkin -- due immediately.

Into this mix is a new an interesting question. Once your contract is over, what if you just kept your old phone?

Earlier this week, I wrote After two years of Android, is it time to switch back to iPhone? After a degree of indecision (there are benefits to both Android and iOS), I tossed out this comment: "Heck, maybe I'll just keep my Galaxy S4 and buy another battery."

I got an unexpected degree of pushback and questions. Some of you wondered what kind of tech journalist I could possibly be if I was even considering keeping a two-year-old phone. But others of you wondered how such a thing would work, and if it was even a viable strategy. The email message that caught my eye, though, was this "Really? I could do that?"

Yes, as it turns out you can. The answer breaks out into two approaches: getting a new phone, but not turning in the old phone -- and just keeping and using your old phone. Let's take those in order.

Keeping your old phone as a spare handheld device

I wish I could tell you what happened to my old Treos. We probably dumped them during one of our moves. But my wife and I still have my old iPhone 3G, my wife's old iPhone 4, and my old iPhone 4S -- in addition to our not-yet-replaced Samsung S4s.

The iPhone 3G actually worked as a phone for a short while when we needed a spare. I ran down to the local AT&T store, picked up a SIM card (which I think was free at the time). Now, the iPhone 3G acts as a Pandora music station in the gym. It's attached to a set of speakers and still works basically as an iPod touch.

My wife's old iPhone 4 is in the puppy's play room. We have it hooked up as a baby monitor, and it sends video to any of our iPads or Macs.

I still use my old iPhone 4S every night (even though the screen is starting to peel off the back, making pressing the Home button an adventure and touch a bit of an exercise in randomness). I use it as my primary Kindle reader (I read before I go to sleep) and as the way I check in with Facebook (which I tend not to do until the end of the day).

All three of these machines are fully functional (well, the 4S is on the edge and when its screen fully falls off, it will need to go). The iPhone 3G is running iOS 6, and the other two are running iOS 7. Since they work fine with the older OS versions and since the few apps they run still work, I saw no reason to take a chance on performance issues with an upgrade.

So how do you connect to the Internet with an old phone? If you no longer pay for a carrier, you'll no longer have cellular or data service. But WiFi will still work just fine. When you look at your phone's signal, you'll get zero bars for the phone service (and, sometimes, an annoying reminder that you're not connected -- I'm looking at you, Windows phones!), but your WiFi will work just fine. I do my nightly reading and Pandora listening and puppy watching all over my in-house WiFi.

The point is, though, that you can definitely keep and make use of your old phones -- they are fully-functional Internet machines in a handheld size. Only your imagination is the limit.

Keeping your old phone as your primary phone

Now, this is a bit more interesting. Getting a new phone is downright expensive -- as much or more than buying a new PC, given most of today's prices. So do you need to march along with the drumbeat of the every-two-year upgrade? Or can you save your Benjamins for another day?

Let's start with the possible. Sure, you can keep your phone. Right now, our two Samsung S4s are past the end of their contract and still on Verizon. Sadly, we're also still paying Verizon's very high rates. I'm just waiting to see the full spectrum of this season's phone announcements (and I'm very interested in the Nexus 6 and Project Fi), so we'll probably wait another month or so.

But you could also switch carriers. The issue has to do with whether the phone you have is compatible. There are some issues with what network you're on, whether CDMA, GSM, LTE or some other variation -- it gets to be alphabet soup after a while. It's best to go into one of the phone stores with your old phone and ask them to look it up and see if it can be used.

To switch, however, you'll need to unlock your phone. Your old carrier should be able (and willing) to unlock your phone after your contract period is up. If not, there are a tremendous number of unlock sources on the Internet. Just watch out, and make sure to avoid scams.

Once you unlock your phone, you can switch carriers. Based on our current Verizon plan, my wife and I could easily save 50 percent or more my switching to T-Mobile or Sprint (both of which support WiFi calling, even on the old phone). While neither has the coverage of Verizon, that's a compelling option.

So, keeping your own phone is practical and possible, from a switching carriers point of view. But is it practical from a daily-driver point of view?

Here's where the practice begins to break down. And it very much depends on your phone. If your old phone was once very popular, you have a chance of still getting updates -- particularly security updates. If your old phone was a sell-it-and-forget-it phone, you may be stuck on an old version of Android (this applies mostly to Android phones) that will never be upgraded.

An un-upgraded Android phone is unsafe for daily use. Period. There are far too many security flaws in Android and the potential of losing your data or having your phone otherwise p0wned is too high. If your phone can't be upgraded, I wouldn't recommend using it as a daily-use phone.

There's a problem with upgrades, though, too. You're adding a lot of layers of upgrades -- and more and more software -- onto your phone. After a while, your phone starts to feel like the tax code. Rather than a clean, straightforward environment, it's patched to an inch of its life, and patches in one area begin to fight with patches in another.

Granted it's not like the tax code in that it will still work and not rip us all off, but eventually, the phone will begin to show signs of slowing down -- possibly a lot. Another reason for the slowdown is upgrades often expect the faster processors of more modern phones, and a two-year wait in technology may be reflected in a considerably slowdown.

The other problem is apps. Some apps will work on older versions of the phone's operating system, but some won't. My old iPhone 3G, for instance, won't upgrade past iOS 6 -- and that means we can't run the Gmail app. It's not really an issue because I use that machine for Pandora, but if I relied on it for daily use, Gmail would be out (and yes, I know there are other mail apps, but you see where I'm going here).

As your phone ages, you will find that more and more apps will either not be compatible, or not supported on older versions of the operating system.

Bottom line

What's the bottom line? You can certainly keep your old phones and put them to use. When I upgrade my phones, I'll probably replace my crumbling iPhone 4S as my nightly reader with my comparably new Samsung S4.

You can also keep and re-carrier your old phones. However, as time passes, upgrades will be harder to get, they will slow down your phone's performance, be subject to more security flaws, and stop running some favorite apps.

My advice is this: keep your old phone for fun and games. Even keep your old phone for a few months (as I am, waiting to see the full spectrum of new offerings). But don't wait much more than a six months or so after your contract ends to get a new phone.

Update: Updated to remove specific CDMA and GSM references and to add suggestion to check with a carrier before switching.

By the way, I'm doing more updates on Twitter and Facebook than ever before. Be sure to follow me on Twitter at @DavidGewirtz and on Facebook at Facebook.com/DavidGewirtz.