I don't believe the Excelsior II was built during the 2340's or whatever when the other Excelsior 'boom' was happening, despite them having 4XXXX registries. I think that was just a quirk of Drexler's, and that those ships were new as of 2401. They don't look anything like ships that were made in that time period, unless they were refits of older Excelsiors, which they clearly aren't.
Excelsior II definitely seems like a refit of the old Excelsiors to me.
I wouldn't be surprised if SF did an intermediary refit of those old Excelsiors (or at least some of them) in the late 24th century which turned them into the Obena class (because of how a lot of new starships in 2380-ies for example had Sovereign style nacelles - which also ties up to what Carol Freeman said she didn't want the Cerritos to be 'snazzed up')... and then they were turned to the Excelsior II by 2401.
That way the originals (or at least those that were still flying around) never went out of service, they were simply repurposed to become modern starships (as its possible the technology changed so much to the point where hull designs had to undergo changes - but in that case, why not just call them the same class of ships instead of turning them into a new class alltogether?).
PIC S3 did introduce a lot of different classes of ships for no apparent reason which at first glance looked like small changes to their immediate 'mid/late 24th century predecessor classes'. They could have just continued to upgrade the ships by changing their hull desigs which would follow the technology evolution for example, but keep the class of ships the same, but instead, they did classify them as a different class of ships.
So, what happened to the Titan could have happened to the old Excelsiors.
A similar thing was inferred to have happened to the USS Stargazer when Picard called it a refit in S2 compared to the original Stargazer he flew. The old Stargazer was just refitted into the new class of ship we saw and got a new registry (but in this particular instance, it was not named the Stargazer-A... it was just Stargazer).
I still think SF has too many classes of ships though for no apparent reason.
There is also a severe lack of Galaxy class ships in the 25th century it seems, but those look like they may have been refitted into the Ross class (or at least the leftovers from the preexisting fleet may have been turned into that new class).
There is obviously a change that occurred which in-universe necessitated this... I think S2 of Picard mentioned integration of Borg technology, which I think could be explained as a reason for this shift.
So instead of continuing with existing classes of ships from the TNG era, because of introduction of Borg technology, the ships had to be redesigned perhaps - but I still think they could have just kept the classes of ships the same and said they were just upgrades... it would have worked either way.
