Your best bet to get this answered is to call .Mac and ask. If they can do it, then they will do it right then and there. Otherwise, its a no. Sorry, but not to many people ask this type of question, but my guess is that they might allow it.
I bought an additional email address (which apparently according to the options I am given, I should be able to convert my sub-account into this new email address).
But when I enter my alias' email address into the box and click 'convert' it comes up with an error message that staes 'invalid email address and password'.
If the second address is one of those £7.50 mail-only accounts, I think you can only make the change at renewal time. If it's one of the aliases they include with the regular .Mac subscription, I don't think you can use the same trick.