I'm not sure it's been tried. It should work to go from stable to testing, but there are no guarantees. Be warned though, there is no easy way back to stable! The path from stable to testing should just be additive. Newer versions should be able to install and figure out their dependencies on their own. Testing to stable is subtractive. And even if you can figure out which packages need to be reverted, you'd have to revert them by hand (CLI) and none of the tools do that quite as good as they probably should. I'm not sure it's worth it just to get codecs to work. There are other options for tweaking you may want to explore first. Edit: I'm also not sure the 720p is even in CSSU Testing. AFAIK it's a separate animal one must apply by hand, even in CSSU-T.