Starfleet might have been ready to put the Connies on reserve fleet status, available but not crewed. They were going (as we know now) to start building a lot of Excelsiors which would remain, along with Mirandas, part of the backbone of starfleet for a long time. Whether the "A" was re-named older ship already out of duty, or put together from "New old stock" parts (rather like NASA's space shuttle Endeavour), it was possibly a stop-gap to cover a need while new heavy cruisers were built.
As it happened they just had a crew with plenty of experience ready to crew one, so why not put them on it for a few years until it was ready for mothballing.
As it happened they just had a crew with plenty of experience ready to crew one, so why not put them on it for a few years until it was ready for mothballing.
