Hello mmfraser.

I believe that DONE3s (the relevant RTW fare for you) are changeable and refundable although perhaps with a fee.

Can I suggest however that you do not book this itinerary yet. You will probably find that about GBP 4000 of the price is the fare component and GBP 1000 or so is fuel surcharges. I believe that the fuel surcharge may fall a significant amount over the course of the next 8 months so you could save a fair bit by booking later. You can book RTW very close to departure.

Disclaimer: Of course, I may be wrong, D class may close etc...