can i change from vanilla to fabric safely

Create a backup first.

Download the worlds and any other data (like playerdata) you want to keep.

Then change to Fabric.

Reupload the worlds and the other data.

Quote from Roman

Your world will only get cleared if the new software is not compatible with the existing world.

Yes and no, there can be problems like world resseting, but probably that not happen. But i recoming you, create a server backup and download world.

Note: Switching from 1.16 to 1.16 or 1.17 to 1.17 is often not a problem. Newer Versions (1.15 - 1.17) to Old Versions (1.14<) is oft world deleting and new-world-creating. Sorry for my bad english. Bye