I expect that it is not possible to upgrade (at least not unless on is top-tier in SU's FFP). Also note that the 77W has a PE cabin, whereas the A330 is 2-class--but from what I understand, the 77W has a superior business class product (I have not yet been on an SU 77W). Thus, you would need a one-class UG on the A330, but a two-class on the 77W (or settle for PE).

I generally find SU Y class reasonably comfortable, though admittedly have not endured it for more than 5 hours for some time. If you got a cheap fare, my advice would be to do whatever keeps you occupied on long-haul flights, save the money/miles from the UG and treat yourself and your GF on arrival.