A young lieutenant’s assignment to turn an antiquated battle cruiser into a traveling museum becomes a life and death struggle when he and his retiree crew must defend a colony world under attack.

Try this one: A lieutenant's assignment to turn an antiquated 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.

Thanks. Your comments helped a lot.

Who is attacking the colony world ?? Why must they defend the colony world ??

Knowing these questions and including them would help us as a reader.
The logline starts of good and then I stopped caring when I got to the end.
Make us cheer for the lieutenant and his crew. Give us a clearer obstacle.

Did the battle cruiser need a new part and they stopped at a colony world to fix it ??
Is the colony world their home ??

Also you can cut down the wording for a much stronger approach.

We know by reading the logline that the battle cruiser is being retired and there is not a need to use "retiree" crew as I thought it meant:

1) The old crew who ran the battle cruiser (Senior Citizens)
2) A crew who's last mission is the assignment

Hope this helps :D