Follow the below steps to convert the items from FBM to FBA or create a new FBA Listing.
Create a new batch
Edit the batch and update the batch to FBA and save.
Add items to the batch.
It will auto start to convert the items or create new listings as FBA .
Wait for the new listings to get created. (The below message will go away from all the items.)
Once the above message disappear from all the items, the FBA listings are created.