Yeah you can keep the same ones you've had from the start.
They can trained with the same abilities as any newer bridge officer. It comes down the the traits that each BO has. The BO's you get from the start have basic traits, and there are better or "superior" traits out there.
I still use my original crew, though I have trained/replaced much of their abilities to be what I want. It does feel weird to have to ditch BO's because some new guy/gal has superior racial traits. Maybe someday when I feel like blowing all my millions of energy credits and hundreds of thousand of merits on repurchasing a new superior only crew and then retraining all of them....ya...not going to happen...
Is it possible to keep and promote your original BOFFs all the way to endgame?
I have two that I have received as rewards but I haven't activated them yet because I have grown attached to my current crew.
In Star Trek shows, Warf wasn't replaced with a new guy every time Picard LvLed up!
Yep you can keep them the whole way, you can also 'upgrade' them by saving the costume of each boff and finding a boff of better quality - better traits-, get one of the same race, sex, and class and copy the old boff costume onto the new, delete the old boff after stripping it, and rename the new boff with the old boff's name. Old boff back, but better. Also, you don't need to limit yourself to just the same 4 all the time, more boffs means more options, use some for space and some for ground, you DO have a large crew after all.
It all depends. You go gain more Commissioned Officers slots as you gain promotions (up to 10, more if you you buy them). The main differences iin BO's are skills and traits. Skills can be gained by training by another BO or Captain. Traits are fixed for now. But the main thing about traits is that it's mostly Ground Traits BO's have.
So you can can add the better/rarer BO's to your ship as you level up. HAving a lot of BO choices allows different load outs for away teams and ship combat configurations (i.e. anti-borg away teams and ship configurations). If you gains a BO that has more desirable traits than a current BO you are close to, you save the costume/appearance at the trails, rename/dismiss the old BO and rename the new BO and change up their costumes. And you won't have to worry about Merits/Honor and BO Skill Points, as you will have more then enough at upper ranks. (Right now my older character could change ot my old BOs 10x over, if I cared to.) And I still have my original BOs around o most of my characters. They will be becoming Department Heads and First Officers when those systems foll out.
I always keep my original crew. What I will do occasionally is save the costume file of a given boff, and replace them with a higher grade of the same species/gender - then load their costume file and the like.
Basically - Same character, better abilities.
That's just how I do it anyway, and I don't even do that most of the time since I'm immensely lazy.