I checked the small print and realised that my desired new date for one of the legs exceeded the one year validity since date of booking. Confirmed with Marco Polo that I needed to cancel both legs under the same booking number and incur a $120 cancellation charge for the redemption cancellation. Will get the 50k points back and start again. Checked and no apparent problem getting dates if I want to repeat. Maybe next time I will get separate booking numbers for each leg to make things simpler, particularly as no difference in points reduction for doing a return.