Hi all, I would like to copy my world from ATM8 to ATM9 but it tells me that the world cannot be loaded because it has experimental data, can I do something about it to make it work? Thank you.

Hey, ATM9 is a new pack. It doesnt support importing your ATM8 world.

So should I start all over again being just a save with pretty much the same mods?

Most of modded minecraft is not able to load worlds created on older versions of minecraft. You are only able to do that with a vanilla (non modified) world. That said, there are various mods (save and load inventories mod is one) that allow you to save items (via a file that has a list of the items) from old worlds, and load them in new worlds. You would need to install the mod yourself.

You could also save your builds with mods, like the create mods schematics canon, so you don't need to totally start new