Should be able to go in, edit both phones in Extension Mapping, then do Save and Rebuild. Downtime should be minimal to just the reboot to apply new template.
Yes - this is how I thought it should work. I tested it on two phones that are not being used at the moment and it doesn’t work. The phones reboot, but still have the same extensions that they had before switching the phone model and MAC address in the endpoint manager.
The phones are the ones that connect to the PBX by extension (the phone requests the extension), so you’ll need to update the config in the phones, then reboot them, and check to see where they connect.
Bricking the phone may be you best option, so drag out the magic button sequence and factory reset those bad boys.