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The reason not to have your account changed from dollars to pounds by Princess is that the exchange rate is not very good, you are better having it left in dollars and have your credit card company change it. To do this when you check in you have to complete a form and at the bottom are 2 boxes, on says convert by Princess the other indicates you can leave it in dollars (sorry I can't exactly remember the wording).
We left ours in dollars and have got a very good rate of exchange when the credit card bill arrived.
Hope you can make sense of this.