Changing the inventory source in your Amazon Seller Central is NOT the correct workflow for this.
Proper Workflow for Adding a New Inventory Source to an ASIN.
1. In your Amazon Seller Central, create a new SKU that points to the same ASIN. It can even be a similar SKU, but perhaps you had a '-FBA' or -'FBM' to the SKU, so at a glance, you know which inventory source it uses.
2. Assign the new inventory source for that listing. If you are creating a new FBM SKU, make sure to set the quantity to what you currently have in your warehouse.
3. This listing will then be imported into Listing Mirror.
4. Listing Mirror will see that it has the same ASIN as the other SKU, and they will be linked as Duplicate Listings.