OK, I know this wouldn't be of use for a full on "Shift the whole planet through seasonal variations" Tileset with all the fixtures and fittings, but...
I'm currently thinking of a sort of triple approach, partly because of all the additional geometry maybe causing lag in a single big Tileset - and partly just a "how little work can I make this" angle...
If I have two Tilesets - let's call them "Normal" and "Apocalypsomized", the latter being being a reworked version of the first where everything's been pushed in with a catastrophically immense Eldritch Brick, both of which are fairly big anyway (being Hyenaomized versions of the TNO set with assorted additions from Q and of my own rubbish making), then putting them both into one set is possibly asking for trouble. And it's an awful lot of work to complete and find it's not really doing the job.
So, perhaps the answer, at least in my ridiculous genocidal desire to annihilate every last thing there is, is to create a third version with just the tiles - and ONLY the tiles - needed for the actual Area in which the actual change takes place. Preceding Areas use the "Normal" and subsequent Areas use the "Apocalypsomized" Tilesets. This keeps the extra geometry to a minimum and they can share walkmeshes since the big heaps of rubble for fallen towers, etc, can be script spawned as needed as Placeables at carefully positioned waypoints.
As I say, this won't help for a seasonally shifting game world - which would be awesome - but it might be the way to go for a sudden, total change of the world around a player, on screen, properly, not cheating...
And not adding anything else to Tilesets already bordering upon the bloated.