Convert Minecraft Client to other versions?

I wrote a Minecraft Client 1.8 in Java, but would like to use it in 1.15, how can you do that?

There are two possibilities I could think of:

First option:

You build in a protocol hack so that you can get your 1.8 client on 1.9 (and higher) server.

Second option:

You put on a 1.15 client and write everything all over again

You create a 1.15 client and copy over the desired functions. But you'll have to change a lot.

So just install 1.15 mod coder pack and copy functions?

Yep, exactly.

There's no simple upgrade function. You have to copy your old code over to the 1.15 MCP project by hand and adapt everything that no longer works.