Change Minecraft version are saved games lost?

I'm currently playing Minecraft java or Optifine and have already built quite a lot in my world and I also use shaders. But now I would like to download mods and for that I need Forge, I also know how to put optifine and Forge together, but the thing is that you have to switch from Optifine to Forge if you understand what I mean. So I mean at the bottom left at the very beginning where you can virtually select the version of Minecraft so Optifine, Forge, the normal mc, etc.

My question now is, if I switch to Forge now, will my worlds that I made in Optifine be lost? Or can I somehow save it so that I have it with Forge?

No, they actually don't. Would save it anyway, because mods (or loading a world into an earlier version) can damage the world if loaded accidentally. If you play with the Forge version, which is also running your normal world, the same applies to the backups.

No, everything stays as it is, you don't have to worry.

Greetings, wolf tongue!

If you have any questions, please let me know!

Here to my https://youtube.com/.../Wolfzunge