The old pass will work. We did this for our first cruise to get credit for it. There was instructions on who to email somewhere on the Carnival website. (Look under past guest FAQ) I think we sent them a photocopy of the pass and that was good enough (since the wife still has them in our honeymoon cruise album.)
When talking to them to get the credit, they also said an embarkation photo would work also.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007