Minecraft Map/Save Game Updating from Modded 1.7.10 to 1.12.2?

Solution 1:

To be honest, this will almost certainly be impossible, if not difficult enough to be not worth it, regardless of how much progress has been made on the world. Many mods changed significantly between 1.7 and 1.12, as has forge itself. Even if all the same mods are present, some of them (Thaumcraft and Thermal [X] to name a couple big ones) have changed so significantly, that there are blocks that were removed entirely and have no new equivalent, and so nothing to convert into.

Depending on how your buildings look, the best bet might be to load it in vanilla to trim out all the modded stuff, but leave buildings in place that used only or mostly vanilla blocks, then cheat in equivalent stuff. Still, this is going to be a giant pain, and requires re-assembling all the factories and farms and stuff by hand.