You can modify Judge assignments at any time- before, during, and after your competition.
RocketJudge signature auto-assignment algorithm kicks in when you click "Check In" next to a Judge's name (or when they click "Check In" themselves for Judge self-check in), so unless you do manual assignments, there will not be assignments to modify before the competition begins/check in starts.
We do not recommend modifying Judge assignments after the Competition ends, because you might accidentally delete Judge evaluations. Deletions are permanent.
Learn how to change judge assignments here.