I've booked a cheap 'N' fare return SIN - HKG. I'm a GO.

Is there any disadvantage in checking how full the flight is, waiting to see if there's an op-up and getting the benefit of GO and then if I'm not upgraded, changing the FF# to BAEC at the desk to get the 25% avios/tier points on BAEC vs 0 on CX MP?

Also just to confim does the class of ticket make (e.g. N vs V) make any difference to op up chances? I think it's just tier then check in sequence but wanted to check.

Thank you all