If you booked through a TA, any changes have to go through the TA. Just this week our TA (who has booked 30+ cruises for us) helped us get a verandah instead of the oceanview we booked based on a new promotion after final payment. He has previously done this for us as well. But in the past, we have lost certain benefits of a prior promotion if we wanted the benefits of a new promotion, so it is necessary to do the math sometimes to see if you come out ahead.
So it is likely they are currently in the process of doing upsells and cabin assignments. Assuming guarantee cabins are assigned last, you are essentially asking them to move you to the front of the line instead of assign the cabin based on their schedule/process. Maybe your travel agent can make that happen, but if not, you will get a cabin assigned eventually.
Best wishes for getting a great cabin!