So, would changing death saves from an untied save to a Constitution check fall in your vision of Level Up? Or are you focused on keeping existing mechanics the same? For instance, I know in the other thread I mentioned stackable advantage/disadvantage. It is the same mechanic (mostly), you just use more of it (so to speak...).
So... here's the way I see it.
A mechanic like that isn't an actual compatibility issue. You're playing
Curse of Strahd using
Level Up rules. The important thing is all the monsters and magic items and stuff in CoS work as written in
Level Up. The fact that you're using a different death save rule doesn't affect the compatibility of CoS. It might change the deadliness of CoS a bit or something (in the sense that many
Level Up is more or less deadly than 5E) but CoS is still fully usable as-is in
Level Up.
A compatibility issue arises when you have to run a monster from CoS and it doesn't work in
Level Up without some conversion work. That's what we want to avoid with backwards compatibility.
Now, if we change a core mechanic, and 5E has character or monster abilities abilities which hinge off that core mechanic, we might run into a compatibility issue. Because the character class or monster is no longer compatible; but it might still work with the new mechanic. But this is detail stuff which is what we're going to be working hard on. And yes, it will very much limit what we can change.
But we're not planning on making sweeping changes to the core game. it will still very recognizably be the same game.
One approach is that we simply rewrite the abilities. The character has that ability, but now it works this way instead. We don't pan to copy/paste any of the SRD, so every word will be our own words, down to the individual spells. If we change how
fireball works, a monster which can cats
fireball can still cast
fireball, just it works a bit different. That's still compatible.