I see, it makes sense – thanks
Did some more digging and found their
official policy on this, which seems good enough:
4.2.1 Once reward tickets are issued, you may amend your travel date, route or class of service prior to travel for an amendment fee of £30 per person for changes to international flights originating in the UK, US$50 per person for changes to international flights originating in the US or local currency equivalent of US$50 per person for changes to flights originating in any other region.
TL;DR: I
can upgrade the class of service of my reward flight, but with a £30 fee and obligatory miles transfer charge.