Originally Posted by
SK AAR
Often there are promotional fares in PE on TG CPH/ARN - BKK - especially in low season like AUG-SEPT. This could be combined with a cash upgrade to C class for BKK-SYD. Should be a much cheaper solution. Please search the TG forum for info on cash upgrades - I think it is around USD 700 each segments - but I'm not really sure.
If flying PE with TG to BKK there is no need to pay for the C upgrade for the SYD leg. You will get it for free

as TG seats Scandinavian PE pax in J-class on connecting TG flights. So to fly TG PE/J to SYD via BKK would also be my recommendation instead of flying SK in Y+ to BKK.
Regarding SK: Their agreement with TG is as follows..
SAS - THAI AIRWAYS
SAS (Economy Extra) Y and S-class fares passengers are entitled to be upgraded on Thai Airways on below code share sectors booked on SK marketing flights between:
BKK-SIN, BKK-KUL, BKK-HKG and BKK-SYD
All passengers booked on SAS Y and S-class fares are entitled to be upgraded to Thai Airways Business Class on above mentioned city pairs.
Must be booked on SK marketing flight number only.
So the agreement is pretty clear. And the SK agent should know that.