Even after knowing how you got into a seat, the response by staff seems to differ. Some consider mileage award or or voucher/upgrade award as deserving of the highest quality service as is a paid ticket. Other staff seem to treat you differently.

My own experiences on AA/UA/CX/BA/SQ, etc. has been very good - they rarely seem to care.

This was true even on some of my CI (China Airlines) upgrades when they had dead cheap upgrades - the checking and gate staff seem to provide "full service" even if you are using their very affordable upgrades. The cabin crew seems to not know the difference.