XO will work on older XS. The main issue could be related to duplicated uuids. Please open a support ticket so we could take a look remotely.
Yes, that's right, since such VMs were imported with "preserve=true" they have the same UUID and XOA shows only one of the two instances (sometime the old one and sometime the new one regardless of running state).
The issue is already known:
Is there a way to change VM UUID without cloning one of the two instances?
Please let me know if it's possible to perform VMs migration from xs 6.2 to a XCP-ng 7/8?
P.S. I don't know where to open a support ticket since I am testing the free version.