I went from PLT to EXP while on a trip to Australia. I hadn't noticed, but on my boarding card it changed from OW Emarald to OW Sapphire (I hope that's the right way around - always get it mixed up!) My details were in the system as well as having a card, but I don't think it made a difference.

When I got the QF lounge in Brisbane, she just looked at my boarding pass and said this way please and took me to a "secret" door into the first class lounge.

Good Luck!