I have an old Tekkit lite world and now want to continue playing with the most important buildings in FTB Infinity Evolved. So i copied them for a test in a FTB-IE world with MC Edit. The game of course crashed. So the problem of course is,
you cant just do that. So now im searching for a tool that could delete ALL modded Blocks and items for me. I tried to replace them with MC-Edit, but that didn't work, because it doesn't display all used block IDs.
(And yes, loosing the modded blocks here wouldn be that worse, thats why i want to delete them all, just to be sure it will work afterwards.)
MCEdit may not list mod blocks but you can manually enter their IDs when doing a find/replace; be sure to check the "all subtypes" box so all data values are replaced.
Also, unknown blocks by themselves usually aren't an issue for the game, which was written to remove unknown blocks when loading a chunk, though if you load a modded world in a different modded version it may incorrectly handle blocks with IDs that appear in both versions.
Unknown entities or items can also cause issues (in particular, the game can delete and regenerate chunks with invalid item entities), again, particularly if they have the same IDs in both modded versions (otherwise, the game appears to safely delete unknown entities, as I once loaded a 1.8 world in 1.6.4 to see what would happen and there were a lot of "skipping entity with id Rabbit" messages and only items caused issues), in which case you can easily delete entities by selecting the region and deleting them; items in chests may be harder to remove without deleting the chests themselves.
Tile entities are another potential issue and many mods add various blocks, including purely decorative blocks, which use them (MCEdit has a "show tile entities" option that highlights them so you can easily find them); again, the issue is likely more due to one mod trying to interpret data from another mod than vanilla itself (note that this can also apply to different versions of the same mod, especially if a lot of time and game versions have passed), which is usually pretty good at handling unknown data (the aforementioned chunk deletion due to invalid items appears to have been fixed in 1.7).