When changing an FBM product to FBA on Amazon, you first need to take off from this product the inventory rule that already exists on Inventory Ahead, in order for this product to be recognized as FBA.
Here is how to do it:
Number 1 – Navigate to the “Listings” tab on the products page.

Go to the item you want to change to FBA, on the “Inventory” column, you will see the source of the inventory rule that is assigned to this item.

Click on the pen next to the existing rule to edit the inventory rule.

Here you will see a list of the already saved inventory rules and also an option of “None”. Click on “None” to take off the current rule from Inventory Ahead. Then click on “save”.

Number 2 – Once you have changed the inventory rule to “None”, you can go to Amazon and change your inventory to 0.
Number 3 – Change your item on Amazon to FBA.
You successfully changed your product to FBA!