1
There is no native method to roll back from the Pro version to the Home version, if the PC manufacturer does not provide an OEM Factory Reset method, check if there is any Asus app installed to perform a factory reset.
If that is not available, then the only way to go back to the Home version is to create a bootable Windows 11 USB an use that to clean install the Home version.
2
When you get a new PC provided that is installed with an activated version of Windows Home you will be able to use your Home to Pro upgrade licence again on the new PC, you will not need to purchase a new Pro upgrade licence.