Friend of mine got an op up Y-J from DXB to LGW last week. He further upgraded to F onboard with miles and was told it would be 39k miles. He was subsequently very surprised to get an email halfway through the flight that 96.5k miles had been deducted instead.

The purser apparently told him that the IT system now automatically deducts the Y-J miles in addition to the J-F miles but in principle there is no formal rule against using miles to upgrade an op up. He wrote to Skywards and the additional miles deducted were refunded within a day.