It adds a lot of stuff and changes the vanilla textures a bit. But more importantly, the terrain generation isn't radically different from previous versions. I tested as far back as 1.9.4, and for the most part everything is the same with the exception of surface details such as trees. Given that most towns are going to have altered land as a matter of course, I think it would be worthwhile to make a new world with the old world's seed and transfer our towns over to it. Any differences in elevation should be easy to fix with a little hands-on terraforming done by players. Of course, this is all dependent on which version the current main world was made in.
Also, automating the mining world regen would be nice too
Also, automating the mining world regen would be nice too