Can you change career in Star Trek Online?

No. Tactical, Engineering or Science is fixed for that character. You can retrain skills, change ships and rebuild your setup, but the career itself does not change.

Direct Answer

Career is permanent

Career choice

If a character was created as Tactical, Engineering or Science, that character stays in that career. To play a different career, you need another character.

Sources checked: STO Wiki: Player captain

What You Can Change

Career is permanent, but skills and ship choice are not

Part of your character	Can it change?	What that means
Career	No	Tactical, Engineering or Science stays with that character.
Skills	Yes, with a retrain	You can retrain and spend skill points differently.
Ship type	Yes	Your career does not lock you to a matching ship type.