I believe I’ve sorted it out now…
(1) All pre-cruise purchases (specialty restaurants, dining packages, drink package, excursions, etc.) that you purchase before the cruise is in the same currency as the currency that was used for the original booking. This cannot be changed. The currency for on-board expense account which you set up at check-in is different. You can change the currency for this account with Guest services once you are on board.
(2) It turns out that the exchange rate that RCI is charging for pre-cruise purchases is actually reasonable. I had miscalculated it because I got the wrong price for the 3-night dining package from the RCI rep (Quantum and Oasis class ships are $80 instead of the quoted $70 which resulted in my 10% error)
If for some reason it turns out that I am unable to change the currency for our on-board account once we are on our way, I will post here.