If a user is using Eden months after Frodo is released he should also just stick to the old Eden compatible remote.
And I also don't think there won't be any new features which are still compatible with Eden. I think it may be possible to make some of the API differences compatible with both versions (most buttons on the keypad are just a single line of code, both for Eden and Frodo). But at least I don't want to clutter the UI code with "if it is Frodo, than display this extra button" code. If it are only a few extra lines of code in the C++ code I will add them. But if both for Eden and Frodo it requires more then, say 10, 20 lines of code, I probably aren't gonna make the change for both versions.
And about the "when to break Eden compatibility?". When we had the discussion a couple of weeks ago mzanetti said he was thinking about "the next version" (after the version we are currently working on) I think. But I also think that would be to soon (as I guess that would still be before the end of the year, when Frodo isn't even final). I would at least like to have a full Eden compatible version till Frodo gets released, without for examples buttons only working in Frodo. Although there may be some nice extras for Frodo users (automatically open keyboard, for one). But these changes shouldn't hinder the Eden users.