So QA only tests with having the primary and UMP on the same platforms.
So officially not supported this usually comes into play because of windows auth for MS SQL server is not well support on linux boxes.
When using MYSQL or oracle back-end there is less of a problem with the database connection.
So if you are using MYSQL or oracle back-end then the switch should not be that big of a deal.
There really is no migration to be done. You would either install a new robot and run the UMP installer or follow the instructions for setting up a new ump as you would for the multi ump installation.
If you have done customization to the portal-ext.properties files
or other custom changes to the file system on the now primary UMP you would want to copy those files to the new servers.
Hope this helps