Krschkr: TR is not even a total conversion, "just" a particularly large and ambitious mod. Morroblivion, Skyblivion and Skywind should rather be compared with OpenMW and Daggerfall Unity, like nightcraw1er.488 said, as these projects try to recreate elder scrolls games in a different engine.
paladin181: They are trying to build new engines for the games. Skyblivion is porting over existing content as a TC mod. I think that building worlds the size of TR from scratch is not terribly different than recreating existing content in the Skyrim engine.. Skyblivion isn't building a new engine, but adding content to an existing one..
Yeah, it’s a bit half and half. Tamriel rebuilt, overhaul, rebuilt, these are all mods which are built to run in the same engine as the game they are modding, skybivion and such like are moving a game to a different engine. But of course, as with openmw which is an engine reimplementation that can run morrowind with these overhauls. So there is actually three types, mods for the game, mods which move the game to a new engine, and engine reimplements.