[Need help ASAP] Change product from FBM to FBA

Hi, I’m new to updating Amazon’s inventory. Yesterday I did some product grouping using a spreadsheet. I checked back today and some FBA products suddenly switched to FBM. I tried to change it back in the back end but Amazon notified me with the message “Merchant SKU already exists. Merchant SKUs cannot be reused, even if a previous offer using the SKU has been deleted. Please use a new SKU to create this offer.“. Please help me understand what could’ve caused them to change to FBM and how can I change my product back to FBA. Thank you.