I don't know how much the upgrade costs for CPH->BKK, but I know how much for ARN->BKK seg.
ARN->BKK:
eco -> eco+: 4700 SEK/one way.
eco+ -> biz: 4700 SEK/one way.
4700 SEK is roughly 500 EUR.
Wow, that is expensive given that upgrade from Y to C is roughly the same on other European sectors. There is certainly big difference between Y+ (old C seats) and new C and I would not pay this amout for an upgrade to Y+ only.