Once Flybuys have been converted into a reward currency (whether it's Flybuys Points, Pumped Fuel Discounts from Flybuys, or New World Dollars), they can't be converted into another currency, or back to Flybuys.

If you have any Flybuys that have not yet reached the threshold to convert into a reward currency, and change currencies, then they will now contribute towards your new currency. Eg if you are earning New World Dollars, and have collected 3 Flybuys towards the 28 Flybuys you would need to get a 5 New World Dollars reward, then you switch to Pumped Fuel Discounts from Flybuys, those 3 Flybuys will stay in your Account and you will only need 1 more to reach the 4 Flybuys required to get a 3c per litre Fuel Discount. If you switch to Flybuys Points then the 3 Flybuys would convert into Flybuys Points immediately.