Changing software without losing world

I made a server using Vanilla as the software and me and my friends played on it and made progress later i wanted to use mods so i changed the software to forge there was no deletion of worlds now i want to change back to vanilla since it was better and i dont need mods but it says my world will be deleted and that i can backup my world. After I backup and change software to vanilla and restore the back the software changes as well so now what i need to know is how do I change my software back to vanilla and keep my world as well

Thanks for the info