How can I downgrade my world without everything I've built gone?
You can simply change the version if you used blocks that don't exist in the lower one, it doesn't work at all
Just make a backup and try out space.
Does not work. Everything is gone because the worlds are generated differently
It is not that easy.
You could try using WorldEdit or McEdit to create a schematic of your buildings, and then insert them in your 1.12 world (a world with the same seed) at the right place.
Schematics are, as far as I know, version-independent. I don't know of any other possibility.
This will not work. At least not if you want to keep the same world 1 to 1. In this direction it is always a bit critical.
