@treebeard If you want a multiboot device, I know only the Volla boot manager from within Volla OS. So, you must first install Volla OS (which might be illegal - I don't know - and deletes your UT) from UBports installer, and then the other OS with the Volla boot manager to some SD card. You will end up with fresh installations and the whole process does not sound simple.
Volla once offered a reflashing service (for money); if interested, check their web site.