I tend to upgrade them in my own account, have them make a Wix account for themselves, then transfer the completed site to them. This way you can get all the domain connections/transfers done easily, and without their participation (except for email verifications). Many clients don’t want to do the upgrade themselves because they are unfamiliar with the dashboard. I also make sure I download and email their invoices to them for their records.
However, Wix has upgraded a few things where you can keep it in your own account, and easily bill the client yearly. I prefer not to do this, so not sure on all the details… just remember being notified about it. If you have someone that wants monthly/yearly upkeep on a site, this might be the way to go.
If you have any more questions, just give me a holler 