Eh, I could buy it just being a graft of new OS onto an older, easily renamed ship. For the record though, I didn't mean to say they cobbled the ship together for Kirk and company - rather that they had the ship laying around already, essentially useless once its testbed function had been completed, and gave it to Kirk.
You may be onto something about Excelsior
not being the Constitution
replacement, but then again maybe not. The crew's reaction to the ship in III seemed to be skepticism of the "Great Experiment," plus Kirk's line about "new minds, fresh ideas..." somewhat implied that this was the thing replacing them.