Hey, I like wtf levels.
I've tried a few of these before - and incredibly, some are still playable and even solvable. I also found a way to create "super-wtf" levels (in Lemmix) where some of the terrain pieces turn to groups of random pixels - sometimes even these are playable too. I'll post pics and instructions in a moment...
Anyway, I think what Giga means by "swapping textures" is leaving the shapes of the terrain pieces the same, but changing the colour patterns on the tiles to make them look like they're from a different tileset. I'm not sure how you'd go about that though, since each tileset has lots of different textures on its various tiles, and you'd somehow have to choose one for each tile that you want to change.
However, I know something that might work, and achieve a similar effect. You could remake a level in another tileset, in such a way that
every pixel of terrain on the remake corresponds to a pixel of terrain in the original level. In other words, rather than just making it functionally similar to the original (like the upper half of minimac's pic), make it
exactly identical. Then what you get is the same level, but with different coloured terrain.
(EDIT: What the heck happened to the level in minimac's pic? The terrain pieces clearly aren't in the same places. I just tried this myself and got the same thing.)
(EDIT2: OK, I figured it out. Terrain objects with IDs that exceed the maximum of the new tileset (crystal in this case) get reset to 0 and shifted to (0,0). All things considered, that's probably a good thing.)
In some cases, this would be difficult, or even impossible with the 400 terrain limit (try replicating "ROCKY ROAD" in Crystal for example
), but it should be doable for smaller levels with straight terrain.