Our friends over at Wired have started a how-to wiki, explaining how to convert one's HD DVDs to Blu-ray - provided you have a Blu-ray burner on hand, and don't mind breaking a law or two (since you'll need to remove the DRM from your HD DVDs before you can do anything else with the content).
I can't speak to whether or not the process actually works - though it sounds entirely plausible - as I don't have a Blu-ray burner on hand (and as I mentioned earlier in the week, have no plans to get a player anytime soon either).
[The short version of the process - connect your player to your PC, remove DRM and copy the files to your hard disc, convert audio and video to Blu-ray-friendly format, burn. An alternative option is to use an HD capture card, then burn.]
And while this is hardly legal advice, I don't have a personal ethical issue with removing DRM from my own HD DVDs so I can watch them on a Blu-ray player. In my opinion, any legal lapse on that front pales in comparison to the metaphysical comeuppance that the HD DVD and Blu-ray camps have coming to them for putting consumers in this position in the first place.
So why my hesitation? Even if you have all the required hardware and software, you're still looking at anywhere from $15 (25GB) to $45 (50GB) a pop for blank Blu-ray media. Factor in all of the time -- and likely agita -- that conversions will take, and it just doesn't seem like a great trade-off. And even if your goal is simply to stick it to the man, Sony's still going to get their licensing cut of the blank Blu-ray discs you're buying.
So if it's an intellectual exercise - go for it. Otherwise, if it were me, I would just suck it up, and buy my movies again - or better yet, wait until they're all available via download.