Can't downgrade worlds to previous mc versions.

Hello!

So I was building for someone, and they asked for a build with only 1.8 blocks in it. I built the build in 1.13.2 on a server, but purely out of 1.8 blocks. When I downloaded the world to my singleplayer and had load it in 1.13.2, I then downgraded my minecraft to 1.12.2 to see if it would load the world there. Sadly... it's not possible. It purely says: "Building Terrain", and this've been the issue for the last 2 hours.

Please help.