Singapore Airlines KrisFlyer - Purchase uprade by segment?

On my return flight next month, my final segment is SIN-BKK. I am on an internet Y fare.

Will they allow me a purchase upgrade to J for one segment?

Seems like an oversold situation is my only other option. I sure missed the SK lounge on the way out..

Basically - no.

They will just recalculate the entire ticket and charge you the difference of between the J class fare and what you paid.

Hardly worth it for a 2 hour flight.

BKK flights hardly get overbooked. And if they do, SQ would rather put you on the next flight than upgrade you if you do not hold PPS status.