A beaker full of death wrote:
TSFS was originally supposed to include Romulans. The producers at some point said, "who ever heard of Romulans," so they changed it to Klingons. They didn't bother changing the ship.
No, since the movie featured Vulcans such as Saavik, Sarek, T'Lar and Young Spock, aliens with slanted eyebrows and pointed ears, there was concern that the general public would be confused if the movie's antagonists were seemingly identical in appearance. They knew the movie's storyline wouldn't permit an elaborate backstory for the origin of Romulans, so it was easier to switch the enemy race to Klingons, especially since a large number of expensive Klingon leather uniforms were in storage since ST:TMP. Since TOS had already established that there had been a tech exchange by the two powers (ie. Romulans using Klingon vessels - mainly because the Season One Romulan vessel had been lost/stolen/broken), there wasn't need for further elaboration.
There was a stage note in the ST III script that wasn't used in the final draft, where Kruge was a Klingon lord who'd stolen a Romulan bird of prey
"Trek buffs will recognize" its "distinctive bird & wing motif".
I have an old blog entry on this here:
Some fans (as well as old FASA manuals) also believed the Klingon Bird-of-Prey was a Romulan design being used by Klingons, presumably as part of the battle cruiser/cloaking device deal.
This is specified in the above script extract.