I think I have been toying around with the ideas you sketch out here, and have failed to find a good way of doing so. From my understanding your proposal is to use differing rules framework for handling the same creature.
The problem I have found is weirdness happening in transitions. Let us make it simpler with a 2 tiered solution: skill challenges that cannot defeat on low levels, standard combat on higher levels. Chances are that when you transition you find either that the opponent become much easier to handle than with skill challenge ("why didn't we just do this before"), or gets brutally more dangerous ("we easily dodged away from this before, but now we cannot get away due to all the movement restraint abilities even if we try to escape").
Getting this right for one transition is hard enough. Getting it right for 2 sound really ambitious.
I don't think it's anywhere near that ambitious.
Remember that the skill challenge and the combat have
completely different goals.
The skill challenge's goal is merely to survive and/or escape. It's the Fellowship running from the Balrog. Is that incompatible with Merry later getting the first stab against the Witch-King, objectively one of the most powerful of Sauron's forces? Doesn't seem to be so to me. Sure, the Witch-King and the Balrog aren't the same class of being, but the conceptual framework still stands.
If you're actually engaging in combat, the goal isn't mere survival, it's
overcoming. Different goals, different process, different results.
All of the others are simply differences of degree, not kind. You generally wouldn't be using literally the exact same statblock 17 times in a row such that the players repeatedly fight literally the same mechanics and then all of a sudden they binary switch over. I mean, you
could do that, but I'm pretty sure it would be really boring for both you and the players by the time you'd hit the 10th fight against literally identical enemies.
Instead, you space it out--a single Bloodcursed Ogre (solo) at level 5, a mere scout sent by its commander. Then,
multiple levels later, you fight a squad of Bloodcursed Ogres (elite) at, say, level 9. Now it's a whole squad, with goblin scouts and conscripted local bandits and (etc.) because they're on a mission. Perhaps the squad might even feature different variants, where it now matters that one is a sapper and another is heavily-armored, because before, simply
being a Bloodcursed Ogre was the most relevant detail. Then, again
multiple levels later, at say level 12, you fight through a stronghold where Bloodcursed Ogres (standard) are the main type of enemy, and thus you see lots of them. And then finally at (say) level 16, you assault their horrendous overlord's keep, and you're cutting through
mere Bloodcursed Ogres (minions) like a scythe through wheat, because now, only the overlord's commanders and underbosses are powerful enough to actually challenge you--and you finish by slaying that demonic overlord, breaking the blood curse and freeing the remaining ogres to whatever life they were leading beforehand.
Yes, it requires that you think carefully. Yes, it means that you can't literally just constantly reuse the same statblock over and over again.
You shouldn't be doing that in any game. It gets horrendously boring to fight the exact same enemy a hundred times over just to gain a level!