Microsoft announced officially plans for this change on January 25. This new cloud-save default is similar to what Microsoft did last year with Known Folder Migration, via which the company is working to get user data off local PCs and stored, instead, in OneDrive consumer and business.
Users still will be able to change this default by choosing a different location by changing the default working folder. (IT admins, won't be able to alter the Office 365 default save behavior, however, Microsoft officials told Redmond Channel Partner.) It's also worth noting that this change also won't affect any kind of defaults already set by administrators and/or company policies.