Well, it doesnt hurt refitting the Excelsior Class with more modern parts. After all, they did it on the Constitution Class and thats canon. Which ship was most used during the Dominion War? Sovereign Class, Akira Class? No... it was the Miranda and Excelsior Classes.
Imagine that... using ship classes that were 80-100 years old to fight in such a crucial war. My idea is that, AFTER the war perhaps they still kept some Excelsior classes in service... after all, its CANON that the original USS Excelsior under command of Admiral Sulu was patroling the cardassian border when Chakotay got his assistance to apply for Starfleet Academy (Chakotays age would have been around 16-17 years old then).
And being as you said, the ship class is so old, perhaps they DID decide to refit a lot of ships and bring them up to more modern standards. Who says, the USS Titan has to be a brand new class? Who says the USS Titan has the latest and most state of the art technology? Is it so hard to believe that perhaps, Captain Riker did take command of a refitted Excelsior Class? He had been offered command of several other ships in the past, which he turned down. Maybe he chose the Excelsior Class, USS Titan because he thought maybe he wouldnt be offered another command in a while. I'm sure there are more people who want to serve as first officer on the flagship of the Federation.
So i'm just gonna go with my assumption that the USS Titan is an existing class, and that it MIGHT have been a refitted Excelsior Class. Speaking of which... why refit an Excelsior Class to be the standards of the USS Lakota? With stronger phasers then a normal Excelsior, and quantum torpedoes. I mean... its an old ship, so why refit it? Get my point?
If you dont like it, you dont have to download it when its done. Simple as that
EDIT: Reply to Starforce, the warpcore is the primary "fuel source" to any starship. Who is to say, that the warpcore wasnt replaced inside this ship with a more modern warpcore, like that of the Intrepid Class or even a Prometheus Class. I'm sure it would get room since there are more then enough decks inside the Excelsior.