First, sorry for my bad english.
I want to put qa installations on differents subdomains like: cars.domain.com, pets.domain.com, etc. And I want to share their users. Same user and login for all subdomains.
Now imagine that in 6 months from now there is a new version of QA, the 1.8, and this new version change the estructure of the table qa_users. I decided to update one of my subdomains, cars.domain.com. If the table qa_users is the same for all, and I have to change because update cars.domiain.com, what about the others domains? they still working or I have to update it all at the same time?