On the Manage Inventory Page, on the right side click the little down arrow next to the word "Edit." Choose Copy Listing. Set up the new copied listing you just created to be the FBA listing.

Now you have 2 listings of the same product (1 FBM, 1 FBA).

Keep selling the FBM listing as you have been all along, while also prepping some units for FBA. Send in the FBA units. It will take somewhere between 1 and infinity weeks for your items to arrive at the warehouse, be received, and be checked in. While you wait, your FBA stock available will sit at zero. Keep selling on the FBM listing while you wait for the FBA inventory to go live.

Once you see the available units for the FBA items become a non-zero number, people will be able to start buying your FBA items.

At that point you can either,

1) delete the FBM listing from your inventory page and leave the FBA listing live (I do not recommend this option)

2) Leave both listings live at the same price (I also don't recommend this because the algorithm seems to get confused and kind of pings the buy box back and forth between them. And the goal of FBA is to remove the FBM labor from your plate, not still have to do FBM on random days)

3) Leave both listings live at the same price, but set your FBM inventory to 0 (Then there is only one active listing for people to buy from. But if the FBA listing sells out before you have more FBA units arriving, you'll either miss out on those sales, or you'll have to manually update the FBM inventory count and sell FBM until the FBA units go live again. This is a fine approach for a few months while you see how fast the FBA items move and whether you want to shift to just FBA for the item.)

4) Leave both listings live but at different prices (Recommended. This way if the FBA sells out, the sales will just shift to the FBM listing. Price the FBM like $.10 higher than the FBA. That should make the algorithm put the FBA listing as the buy box. And if the FBA units sell out, the sales will roll right to the FBM item without you having to do anything. You will have FBM orders to handle but only when the FBA stock goes to 0. This works well while you work out how many units will sell FBA per month and figure out your shipping schedule to be sure FBA stays in stock, and determine how many lost items you'll have to eat the cost for per month, and determine how many returns of used items you can't resell you'll have to eat the cost for per month. Once you have your FBA listing selling consistently and your FBA stock replenishing at the appropriate times consistently, and you have accounted for the cost of lost and used FBA items appropriately, you can delete the FBM listing.)