You can train bridge officers in another specialization but you cannot change their active specialization. Nothing happens. They stay on their first specialization.
Here's a Reddit post from 4 years ago as of this post.
https://www.reddit.com/r/sto/comments/j6hyqu/unable_to_change_active_specializations_on_my_boff/
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Update:
- Logged in/out. Rebooted the game. Didn't change anything.
- Played around with several different characters and went back to the affected character. Something must have happened during this time because I was now able to switch the active specialization.
Comments
Specialisations can be switched around on social maps, but are 'locked in' during missions, patrols, TFO's, etc.