How to convert your world from Spigot to Vanilla.
step 1.
Stop your server.
Step 2.
using an SFTP client such as FileZilla, open the world you are wanting to convert.
step 3.
if you are converting the nether world, navigate to the /world_nether/DIM1 folder.
if the end world navigate to /world_the_end/DIM1 folder.
step 4.
Now download the region that is inside the folder of your choice.
step 5.
now head to the world folder.
step 6.
upload the region folder that you had downloaded previously from the world of your choice.
And that’s it you have now converted your world from spigot to vanilla!