I think some blocks might transform into others. I'm not sure.
Once I opened a test FTB world on MCEdit and my solar arrays turned into beacons, cable turned into pumpkins, pipes turned into grass and machines into cobwebs. The world was fine though.
I'm not sure what would happen in this situation, though.
EDIT: I just tested this: I made a superflat world, put down some IndustrialCraft blocks down (also a stirling engine and a gravel block just as control blocks), moved the world to the Magic World pack, and (this was quite surprising to me) they just disappeared. They became air blocks. Hope this helps!
Yea, that's kind of what I figured would happen. The only reason that Mcedit uses weird textures is because it only knows the code for vanilla blocks, and doesn't have a texture for modded items. Since it doesn't have them, it just substitutes them for what you are seeing. It does know something's there, though, so it shows it as such. In the case for the world, according to what you are saying, it is similar. Instead of substituting the texture, it just deletes the block, since otherwise would most likely crash the game. Good to know, and thanks!