Hi @Dinger simply claim the new AP into your Dashboard org under Organization > Inventory (claim button) and then remove the old AP from the wireless network where it currently lives, and then add the new one into it.
When you remove the old AP from the existing wireless network, it will still be in the Organization > Inventory, but it will not be counting against the number of licenses in use. If it's a bad AP and is going to be RMA'd for example, you can also "unclaim" it, also from the Organization > Inventory page and that will actually remove it from your Dashboard organization.
To remove the old AP, go to Wireless > Access Points, select the AP, drop down the "Edit" button and select Remove from network.
In other words, a license it never tied to the serial number of an AP for example. So you are not actually moving the license from one AP to another. You're simply licenses for X number of APs, so when you remove the old AP, you are temporarily using X-1 AP licenses until you add the new one back in.
Hope that helps! Let me know if anything still unclear.