Could I ask why you’ve already paid off £2k before the due date? Certainly something I’d never recommend unless of course a T&C of the TA.
Also, despite having used TA’s in the past I very rarely do now as it’s so much easier to make amendments direct and the savings by going with a TA are not necessarily as great as they once were.
With regard to your current dilemma you can definitely cancel and rebook something and be quids in but you might lose your current cabin. I’ve certainly done this a few times in the past but only ever with direct bookings and mostly with Princess.