Here's some stuff from http://en.memory-alpha.org/wiki/USS_Enterprise_(NCC-1701-A)#Background_information about the NCC-1701-A:
While the ship's history before its recommissioning as Enterprise has never been officially stated, several non-canon sources (such as the AMT/Ertl Model kit documentation) have claimed it to formerly be the USS Yorktown (NCC-1717); others cite it as the newly built (but not yet commissioned) former USS Ti-Ho (NCC-1798), or the also newly built USS Atlantis. Captain Scott alludes to the ship being newly built in the ship's log of Star Trek V: The Final Frontier, stating "This new ship must have been built by monkeys".
Mr. Scott's Guide to the Enterprise, released shortly after Star Trek IV: The Voyage Home, cites the origin of NCC-1701-A as the USS Ti-Ho (NCC-1798), an Constitution-class II starship which was a test bed for transwarp technology alongside the USS Excelsior. The Ti-Ho was rechristened Enterprise after Kirk and his crew were exonerated.
I tend to go with the Ti-Ho explanation.