Actually, the reason CaptAdam uses is the "only" answer, and not bullsh*t at all - it's a TV show with a limited budget. Reuse the models, the sets, the costumes, the props - sometimes you dress them up to manke 'em newer, and sometimes the set or prop in question got beat up in storage so it has to be redone. Hell's bells, the movie 1701 had 4 different bridges, and the time difference between the one in STIV and STV is what, maybe 3 months, tops? The movies, even the JJ ones that had truckloads of $$, had to save money where they can.
It's totally fun to try to rationalize these details in a believable way, but, you have to always refer to that fact above.
There plenty of real world examples of "older frames with newer guts." I'm sure there's plenty of decent examples in the military. My own anecdote - I have a '75 postal jeep that has a rebuilt trans from a 90's car, and new seats from a 2008 minivan. I bet i could even put some nice LCD displays in it to make it look "modern," but it's still a 40 year old car. Ooo, maybe I could put in a HUD! hee.
For the record Adam, the oldest trains in the NYC subway system are from 1964, and about 1/6 of the fleet was made between 1964-1978, and maybe 3/4 (or more) are from 1999 on. Here in Philly, one subway line's cars was replaced in the 80s, and the other '97-99. Trains aren't cheap to make (and there has been a sad trend in the last 35 years in this country to spend less on infrastructre in favor of corporate tax breaks), so they're expected to last 25 years or more. I'm not a huge train buff but I do like subways, both riding them and using them in model train setups.