Being that you can only merge one "Classic" account with one "BOINC" account, and seeing that the account you're posing with is from 2001 which places it firmly in the "Classic" camp, I'm guessing you've already merged this account you've logged into with 1 of your Classic accounts.
Unfortunately, additional accounts cannot be merged. The Project Administrators did not gather any uniquely identifying information about us except the email address itself, so no matter what information is provided to a Project Administrator, they would have no way of verifying this information and connecting it back to you well enough to say "yep, this is that person".
As disappointing as that is, all you retain from your old Classic account is your start date. The workunits you completed are held as a separate stat and are not converted to credits, so you're going to start fresh anyway. May as well get to work and start collecting new credits! :-)