In concurrence to what Spack has stated, the AB40k files are designed to be as strictly 'legal' as possible. So when a psyker is required to make a choice of what powers it will have, this is dealt with in AB to keep your list as legal as possible. The idea is to have completely finished battle ready list, with nothing left to do before you deploy your army.
If you want to tailor to your opponent, the simplest solution is to create multiple lists and bring them all along. (If I were to tailor for an opponent the psychic powers would not be my only change
)
On a side note, if you are planning for a tournament:
Be aware that some tournament organizers will not let you modify your list(including which psychic powers you have) in between games. I have seen this technique attempted/argued in a tourney before and the guy got ejected from the tourney for cheating
.
So a word from the wise, save yourself the problem and check with your tourney organizer before you open a can of worms too!