Amazing. Three years gone and no solution? At the most services the username is the unique identifier for an account. That means usually you can change your password but not your username. So "Changing user name at other projects" is no go.
Well, that's not how it works with BOINC. Email address is used by the projects to identify your account, at least from the web and account management standpoint. Obviously, you haven't had a look at the "Main account page", where you can change your username, as easily as you can change your email address, password or other information. Because you have all of your other (non-WCG) projects connected to BAM, it's one-stop shopping. Then, you can add your WCG account.
Why is'nt it possible to ask the user to type in user name and password for the "other projects" as soon as you get an error message at signing up?
A question you would have to ask the developers. GridRepublic does it the same way, though with less user input--if it can't find an account with the same username at a particular project, it will just create an account for you without asking.
Even tough I have no clue about how bam communicates with projects this should not be a big deal. A big deal would be to enable boinc to upload the client adjustments to bam, but that's another story ;-).
If you have no clue how any of this works, then how can you say if it is or isn't a "big deal"? To what "client adjustments" are you referring?