Or, for that matter, Skyrim-in-Morrowind?

I am curious to see if there has been any attempt to port the later Elder Scrolls games to the Morrowind engine.

I am also curious about how much would have to be changed for this to work.

(Note: Before anyone mentions Morroblivion or Skywind, keep in mind that those mods port Morrowind to later game engines; I am interested in the reverse.)

There are no direct ports of these games, and that would not be possible anyway - there are too many differences in Scripting, AI, etc.... but there are attempts to create these provinces as they were described in the Morrowind lore (which somewhat differs from what we have in the later games).

They are consolidated as Project Tamriel.

Unfortunately, none of its sub-projects appears to be anywhere near completion.

Also, even if it were possible to port Oblivion or Skyrim back to the Morrowind Engine, it would not be allowed, because it is not permitted to use assets from one of Bethesda's games in another.

Perhaps you could elaborate on these differences?

There are many, too many to list them here and I would not even know them all... also I am not very familiar with the scripting languages of any of these games - though even if they are probably based on one another, there were new functions introduced with every new game that simply cannot be done in the older engine - or only with impractical workarounds, I'd imagine. But for one very obvious example, Morrowind does not have physics, which both Oblivion and Skyrim have, and these games use it for traps and other things, so there is one thing that's not possible in Morrowind.
Another thing is the AI, especially NPC behavior. In Morrowind, NPCs either stand in one place and never move or wander around in one place, they do not have daily schedules, do not sleep, don't enter or leave buildings, and so on. In Oblivion and Skyrim, NPCs have schedules, go around in towns, visit shops, temples, taverns, move from town to town. They even talk to other NPCs. None of this is possible in Morrowind - several modders have tried making NPC schedules, but the results were always impractical and not very impressive. Also, Morrowind has a very limited animation system, so it is not even possible to implement sleeping and sitting animations for NPCs.

These are just a few examples, there is much, much more. The bottom line is, it is simply not possible to port them, and it probably never will, and even if it were possible, it would not be allowed. Some people seem to believe that OpenMW, the open source recreation of Morrowind's engine will some day also support Oblivion and possibly Skyrim, but it would be a lot of work to make it happen, if it is possible at all. Don't expect to see it any time soon.

A related question: Is Skyrim-in-Oblivion technically feasible? If not, why not?

(The "why not" part is what I am particularly interested in.)

I don't know for sure, but I guess the answer is also no.The engine and it's functions were expanded for every new game (Morrowind - Oblivion - Fallout 3 - Skyrim), and while most functions of the previous games were still present or could be emulated (see Morroblivion/Skywind - though both also have their problems with certain aspects of Morrowind, for example some weapon types - and their animations - that just don't exist in the later games, like spears), each new game most certainly introduced things that would not have been possible with the previous version of the engine.

Well, I guess it should be technically possible, but probably not feasible, to port or recreate the Oblivion and Skyrim worlds in Morrowind, including scenery, towns, NPC's, factions etc (I'm not sure about dungeons, which are quite simple in Morrowind - but I'm not familiar with the Morrowind Construction Set). Also all or most of the events managing the respective storylines, quests, etc.
That said, you would have to accept that certain mechanics of the Oblivion and Skyrim engines simply wouldn't work or work differently in the Morrowind engine (and how this affects, for example, scripting, I don't know). A couple of them have been mentioned already (NPC schedules, for example), others affect character development, fighting etc. I myself don't think the latter are always a disadvantage (as, in my mind, Oblivion and, even more, Skyrim have been dumbed down quite a bit compared with Morrowind).
The legal aspect has been mentioned already. It's certainly illegal (unless Bethesda approves, which is unlikely) to copy graphics or other stuff from Oblivion or Skyrim straight or in a slightly modified form into Morrowind mods. On the other hand, it is legal to re-use Morrowind stuff in a Morrowind mod. If you can live with that, fine. Otherwise, I guess the effort to recreate everything yourself is probably prohibitive (I'm also not sure if it's legal to recreate, say, an Imperial which looks exactly like an Oblivion Imperial, from scratch yourself).
Have fun!