I'm not sure the how and the when, but I recall that at some point the intent was to have the Excelsior be renamed the new Enterprise at the end of STIV*. If that idea originated in STIII then I wouldn't be surprised if Nimoy stipulated that the Excelsior look similar enough to the Enterprise to be recognisable.
Given that the early concepts show a more radical design, I'm willing to bet the idea came later, or was simply not stated in the initial design brief.
* Indeed, if you look at the footage as shot, that could well have been the intent right up until post production. The crew's dialogue and reactions are just as consistent with seeing the Excelsior with the Ent's name painted on it as what we ended up with. All they needed to change was the model show. I'd be curious to know if that was indeed the case.