Yes you can do that. Don't forget that you have 30 days to transfer to a TA and it can't be past final payment or paid in full. Group rates get recalled at 120 days prior to sailing. Getting the group rate is not automatic, the TA needs to move it into the group and then call and get the group rate applied. If this was a book on board at the reduced deposit you will have to top up your deposit to the full deposit amount as groups does not accept the lower next cruise deposit.