Theme selection across Pro Sites levels


What I want to do is set up a second Pro Sites level to include additional themes and more disk space.

But there does not seem to be a way to indicate that a theme is to be available on All or Multiple Pro Sites levels. I can only select None or one level.

What happens when a person is using a theme “x” at one Pro Site level wants to upgrade to another that provides more space? It would seem that theme “x” would no longer be available.