I've created a customized world in 1.12 but now I cannot play it in 18w08b, because the support for customized worlds has dropped in 1.13 snapshots.

Error message:
Unfortunately, we cannot support Customized Worlds in this version of Minecraft. Please downgrade to 1.12.2 to play this world. We are sorry for the inconvenience

What I expected to happen:
There should be an option/a feature/tool to convert customized 1.12 worlds to normal worlds in order to keep buildings. I know that the customized features will be lost, but I just want to open the world that I've been playing for a long time.