Back in the days before 241OCU was destroyed, we started doing 'C-to-K' courses for 10 Sqn's first AAR crews as their C1s were converted to C1Ks.
Which meant we had to convert back to the VC10C1, as well as flying the VC10K2, K3 and anticipating the delivery of the K4. There were minor differences between the C1 and C1K, but the checklists and SOPs were different for the -C and the -K. As if that wasn't bad enough, the simulators weren't fully representative of either variant.
At least the checklists and SOPs were subsequently harmonised though, as far as possible.
Then the decision was made to metricate all VC10C1 aircraft. So at one point we had to cope with some which were metric and others which were imperial; I'm not sure, but I don't think that the metrication programme was part of the C1K conversion? At least all the K2/3/4 were metric.
I'm still amazed that there wasn't a serious lb / kg cock-up. If a lb load turned up for a metric aircraft on an AT/AAR task, we had to do the conversion manually - on the flight deck with a 'chocks time' to make... Which was great if the load sheet arrived late. Fortunately everyone coped but eventually all VC10s ended up as either tanker transports or pure tankers - all with metric units. But we also had to cope with receivers using lb, or kg - or even litres, in the case of the Mirage F1. It didn't help when, for example, an F-4 would ask for '4K' meaning 4000 lb rather than 4000 kg - the quaintly British unit of the 'kilopound' being the normal unit used by our AD mates at the time!