NBT tags from Minecraft 1.12 blocks?

I would like to "move" buildings from 1.15.2 to 1.12.2. I do this with the help of the structure blocks, since many NBT days have changed between 1.15.2 and 1.12.2, I edit the .NBT files with the NBTExplorer. Is there a list of all NBT days from 1.12 somewhere or can you just look it up in-game?

Wouldn't it be easier to copy the 1.12 world and update it to a 1.15 world and then copy the structures there?

I want from 1.15 to 1.12, when I open the 1.15 world in 1.12 Minecraft crashes.