@Poyraz Sivrikaya, How do you get into the region files like that? I have never found out how. Also does this mean you could just change the name of the old structures to the new structures and they should work?
Ya I think I am right. I loaded up a world in 20w21a and located a monument. I loaded all the monument chunks and then exited the world. I then loaded that same world in 20w20b where any monument would be known as 'Monument' and killed all the guardians. Not surprisingly no guardians started spawning at all. The monument loaded in 20w21a should be known as 'monument' and the game doesn't spawn guardians in 'monument' and only 'Monument'.
One thing that might help is that I think that the structures are still specified by their capitalized name in the files. I haven't checked to be sure but this might give some insight on the issue. The new name for all structures are in lowercase and so are the bounding boxes of the structures. Minecraft makes things like guardians spawn in 'monument' and not 'Monument'. I dont know if this is entirely true but it might be helpful.
Whenever I try to use NBT explorer and find a chunk, it always takes me to a nether chunk. When I search that same chunk info in the overworld region file, it crashes. Also wouldn't it be easier for Mojang to make minecraft have Monument and monument both valid spawning structures.