I just replaced my Wife's 2010 iMac with a new 4K iMac and used the setup MigrationAssist to restore the environment. After doing this, I see the new mouse and keyboard as connected in BluetoothPreferences, as well as the old named keyboard and mouse.
Right-clicking on the old names only brings up the option to "connect" along with the address. Clicking on the "X" at the right then attempts to Pair the no-longer available keyboard and mouse. There seems to be no way from this screen to delete the old entries for mouse and keyboard.