Never tried with Symantec, but with Veritas, you could contact Support, provide the serials and ask for prior version serials to match
If that doesn't work, return the v11d serials and see if you can find any v9.1 serials on EBay or Craig's List
Those are about the only places you'll find v9.x serials available now
or since v9.1 has been end-of-life for more than a year now, you could just bite the bullet and upgrade