I would like to convert a Minecraft world that I built with version 1.14 so that I can continue to build it with game version 1.8 without major losses. I used extra blocks in my world that are available in 1.8 and didn't use newer blocks. Is it possible to somehow convert this world so that you can play and continue to build it with 1.8 without losing any structures etc?
You can play the launcher in 1.8 and start the world, which is then marked in red.
Everything that was no longer in play after 1.8 will be gone. If technical changes have been made since then (yes, this has been done very frequently), further problems may arise.
Very important: make a copy of your world beforehand and then start the copy. Then no matter what happens to the copy.
It's unfortunately not possible. I have already asked a few questions about it and everyone has said to me that it is not possible. But if you do this on one server, you can create another server and then connect both to BungeeCord! Then one server can be 1.14 and the other 1.8.
