since webuser is not manageruser, what if there is a webuser with username BILL and a manageruser with username BILL. What will happan?
The merge routine (during upgrade) will have to make a decision. If they have the same email address, it is probably safe to assume that they can be combined. If not, then I propose that the manager username should have "manager" appended to it (assuming that this itself does not cause a clash). The admin should be warned that this is the case, and he can then modify stuff (or contact the manager user) accordingly.
This does, in turn, mean that if you are letting random web users sign up it will be worth having duplicate webuser copies of all your manager users, including "admin".