Sounds like she would have let you 'rebook' the room, but when you rebook you do pay the current price. What you are doing when you 'rebook' is booking a new room and then canceling the old one (when the old one is canceled the room number is put onhold until the TA can then assign the newly open room to the newly booked reservation).
So when we 'rebooked' onboard, we were given a new room number and then when we got home our TA took care of canceling the old reservation and getting our room assigned the new reservastion.
If the price had gone up on that catagory, we would have had to pay more for it too. It hadn't gone up at that time (we sail in September, and those prices don't change that much because it is an off season time).
So what I am trying to say is that she didn't say 'no' to you. She would have done what she did for us, it is just our room prices didn't go up so it was worth it to us to go through the process. She was pointing out to you that if you did this, you would be paying more than you would be saving by making the new reservation.
I had a friend that also tried to book after sailing with no luck. They are VERY firm about this. The first year we sailed, they gave us 30 days to book with the onboard credit deal, but now they want you to do it before you leave. I wish they would go back to the 30 days thing so you don't have to use valuable onboard time to book your next cruise.
Oh well...
DJ