A lieutenant’s assignment to turn an antiquated battle cruiser into a traveling museum is interrupted when he and members of the original crew are the only ones in range when a colony is attacked by alien raiders.

Definitely sounds interesting (reminds me of the set up to the latest Battlestar Galactica TV series). I'm starved for good military sci-fi films.
I think what's most interesting about the premise is the tired old soldiers, who think it's time to take it easy, suddenly having to draw from their energy reserves and get back into the fight. Is the lieutenant the best protagonist's POV to tell this story from? Not the retiree commander? Or even just the museum curator?

I like it. It works for me. I'm not too big on the title. It feels overused, especially for such an interesting story as yours. "Old Soldiers Never Die" evokes General Macarthur's speech (even though it predates him).