Greg Cox wrote:
Yeah, I'm not sure changes in art direction (or casting) need an in-universe explanation.
Right, any more than Marvel Comics needs to explain why Peter Parker's face looks different in an issue drawn by John Romita, Jr. than it looks in an issue drawn by Humberto Ramos. It's just interpretation.
For that matter, it doesn't need to be explained why everything in the Trek universe went from looking like live-action in TOS to looking like cartoons in TAS.
In "Space Seed", our heroes thought that the way to study the sleeping leader was to set his cryo-facility to thaw right there and then. If the facility were a removable pod, then the obvious choice would have been to move the pod with Khan still inside to the Enterprise for closer study.
You're forgetting the events of the episode.
MCCOY: We've triggered something, all right. His heart beat's increasing. Now passing eight beats per minute. There are some signs of respiration beginning.
SCOTT: This one was probably programmed to be triggered first.
KIRK: Could he be the leader? The leader. Lieutenant?
MARLA: (dragging herself back from just gazing at the man) Yes, sir. The leader was often set to revive first. This would allow him to decide whether the conditions warranted revival of the others.
They didn't choose to awaken Khan; that happened automatically as soon as they came aboard. So the same would've happened even if the pods were removable.
In any case, apart from the faces of the heroes, these pods would be our very first thing that has been seen both in the new movies and the original continuity. Everything else in the movies (not counting things never seen in the original continuity and thus presenting no continuity problems anyway) is from an alternate timeline, arguably manufactured well after Nero stirred the timestream - up to and including the skyline of San Francisco!
True, the Kelvin
is from an era we never actually saw onscreen, but its reasonable to assume that its technology designs represent a more modern, sophisticated interpretation of what pilot-era technology would have "really" looked like.
There's also Vulcan. The city shown in the film was presumably ShiKahr, but the architecture and the surrounding landscape didn't match what we saw in "Yesteryear." True, they filmed at Vasquez Rocks just as The Voyage Home
did for Vulcan in at least one shot, but they multiplied and exaggerated the Vasquez Rocks cliff by about a thousand.