What's hard for me to understand is your insistence that changing the prefix code must be hard to do. I would think the two senior officers would have the authority and ability to change the security measures of their ship. Is that really that unreasonable? Of course, Spock didn't know that Terrell and Chekov were under Khan's thumb, so he must think Khan is able to change the code on his own. In my view, that means it must not be too difficult, because if it merely takes intelligence to do it, then it can't be a brute force operation, or take a long time. It must be possible to change the code in the time that Reliant was last known to be under Terrell's control.
If I understand your position, you think that because Spock considered Khan intelligent enough to change the code, it must be very difficult to do. I don't see it that way. Sometimes intelligence is as simple as knowing to ask a question and not let ego or emotion get in the way. Khan was known to be at least smart enough to look things up, since he did so in "Space Seed".
I would appreciate you not taking pot shots at me by asking inane questions about whether I read my posts or not. You're new, so maybe you don't know that making things personal isn't a good idea here. Nothing here is that important to take personally. I didn't insult you, you know.