Oh well. I still have my connie, defiant and enterprise D
If you purchased the Excelsior with Marks of Exploration then it was a one time unlock, to reclaim it you'll have to pay the 500 Marks again. If you purchased it from the CStore then it can only be reclaimed from the CStore, the Excelsior in the shipyard is the 500 Marks unlock.
If it's not listed in the Federation Class section of the Ships page of the CStore then it should be in the Special Unlocks area, that's where I usually reclaim mine from.