..which we're led to understand begins only a couple of weeks after the end of TVH
...But then again, it might be more like months. They have to swap the bridge module, after all!

Why did the ST4 costumers fuck up? It doesn't sound like something they would do lightly and without consideration, costuming a main character in a sequel. Especially after getting two sequels more or less right already. Did they just read the first half of the costuming instructions for ST3, which of course begin with "Doohan wears item 24B, the Commander pin"?
Timo Saloniemi
) thought having Scotty be a Captain (when Spock is also a Captain, and Kirk is an Admiral who gets busted down to Captain at the end) would be confusing to folks who hadn't seen The Search for Spock. In the grand scheme of things, it's something most people wouldn't notice. And in the grand scheme of costuming mistakes, it doesn't even come close to that horrendous mix-and-match nightmare that was Lt. (Commander?) Valeris, the half-cadet.
It's only a movie.