In my opinion, when u book early the TA should guarantee you benefit from any price reductions which occur. Your TA is telling u the promo is for new bookings only. U are outside of the penalty time for cancelling.... As long as the new advertised price with credits is now higher than u are paying, simply cancel and rebook. Voila, a new booking. If your TA isnt co operative, find one who is.
Most good TAs will monitor prices for the clients as the sailing date nears, and pass on reductions.