There is no supported way to "migrate" items from one user domain to another, the user domain is part of the uniqueness of the items in the user domain.
1) You can rename a domain per the documentation:
This would still leave the devices in the same domain, it would just be renamed.
2) You could create a new domain, delete the plugin, and configure a new plugin in the new user domain.
THIS WOULD REMOVE ALL EXISTING DEVICES/COMPONENTS AND HISTORICAL DATA FOR THE ORIGINAL ITEMS AND DISCOVER ANEW.