if the Enterprise A is actually a new ship, why they bother to build a new constitution refit class when the Excelsior is already available?
At the time (ST3-5), the Excelsior was still experimental and until that class proved itself the Enterprise's and Reliant's would've been the go-to types. It could very well be the Enterprise-A was the last of a new construction batch that was ordered and re-named at the last minute from whatever previously assigned name to the E-A.
It's not unheard of for such things to happen in Real Life and it seems like it happened in DS9 where the Sao Paolo was re-named the Defiant.
It's possible, but less likely, that the crippled Yorktown was the donor ship as dialogue seems to support the E-A as a new build versus a repaired ship, IMHO.