Hmmm...
Reading this post, and taking the fact that, even as Mr. Roddenberry said, "the only canon material is what is seen on-screen", we can infer this.
The NCC-1701, as shown on the TV series, is a Starship class.
The NCC-1701, refit and as shown in the first 3 movies, is the Enterprise class. Possibly so renamed due to the fact that many newer "starships" of different designs were being planned / built at the time the refit began / ended. Excelsior, Miranda / Reliant, Oberth, etc.
The NCC-1701-A, a replacement for the "Ehterprise" class-refitted Starship class NCC-1701 is in fact the "Constitution" class variant of the Enterprise we've always looked for. Recall, she's different from both prior incarnations of Enterprise (Mr Scott: "I know this ship like the back of my hand". Klunk.)
Another possible thorn in the side of all this. There's a site with numerous
http://www.trekplace.com/interviews/franzjoseph.shtml"" target="_blank">Franz Joeseph interviews that says for most of the time period from when the tech manual started to be generated till the signing of the first movie contract, the Star Fleet Technical Manual was to be the "canon" technical reference of TOS. Therefore, Roddenberry had supposedly seen and signed off on the "Constitition" class as being the ship class all along. Read all the interviews there, for fans of the Tech Manual, it's an interesting history...