So I had an interesting experience recently when trying to apply a GUF2 to an open-jaw booking originating in the USA. I had checked EF and there was I class availability for all the sectors so I booked the WT+ flights and then called BA to upgrade to Club. There were no issues on the outbound flights but in spite of EF showing I class on both the inbound flights the agent couldn't see any for one of the two sectors. So he then proceeded to book the short-haul section into U class as there were Avios redemptions available for that flight. This was despite the original booking being cash, not Avios. I'm not complaining but I wasn't aware it was possible to effectively mix and match the GUF2 upgrades between both revenue and reward classes!