I've been struggling with this issue for a bit as we move toward converting our farm toward a multitenant environment. I was able to make a copy of my current UPS DBs, then use those to create a partitioned UPS add then two tenants.
All the profile info transferred fine (skills, yada yada...), but when I click on OneDrive, I get a "could not load user profile" error. Now, a new user can create his or her own OneDrive, but those who had existing ones are not able to access theirs.
Looking at the logs, I get a "Value cannot be null. Parameter name: userProfileApplicationProxy" exception.
Any thoughts on how I can successfully migrate to the new environment? We definitely would prefer to use multitenancy rather than set up two separate user profile services in two different VMs.
(Oh, and for the record... yes, I moved everything to a dev environment, so my hair is not on fire... yet. :) )