Your problem is a lack of understanding how EPM works.
When you use the update device button, a SIP NOTIFY message is sent to the extension.
Obviously, if you delete a mapping for Extension 123, but do not reset the phone itself, the phone will continue to be provisioned as extension 123.
When you reuse the MAC address for extension 124, which is not currently registered, there is no endpoint to send the SIP NOTIFY to.
Basically, EPM is using this asterisk command (assuming you have a Yealink phone) to tell extension 124 to check its config. But since 124 is not yet registered, nothing answers the message.
pjsip send notify reboot-yealink endpoint 124
There are various contexts for different manufacturers
You can manually tell extension 123 to check it's config (which will make the device check, and pull the new config file with extension 124.
You can see it not work since I have no 124 on this PBX.
But for an extension that exists...
You can see the notify message in sngrep