Hi guys,
sorry for the misunderstanding in providing that information.
here's what happened:
i've upgraded from 2cards gold to 2 card platinum in Nov 2014 (my GST renewal is in Nov too). So the GST for the 2cards gold is waived for 2015 as i canceled it on the spot. Now in view of my new 2 cards platinum, the amex platinum's RM50 was waived but the mastercard was charged rm50 (aka prepaid from Nov 2014 onwards)
basically, it is best to renew/upgrade on the month where the GST renewal will be charge to be "used" at maximum benefit as depict in the situations below:
situation 1:
- gst renewal in Nov 2014
- upgrade cards in Nov 2014
- only pay the RM50 imposed on the new cards and the RM50 of the old cards will be waived
situation 2:
- gst renewal in Nov 2014
- pay the GST renewal of rm50 in Nov 2014
- apply for upgrade in March 2015
- pay RM50 gst for the new cards.
in this case, situation 2 would be the worst case scenario whereby the card holder would be paying gst twice.
Disclaimer: I do not have the proper info as to whether the information that I have shared are legitimate or reliable. It would be best to contact MBB cardcentres/CS for the right information.
sorry for the misunderstanding again (T_T)

I can feedback based on my upgrade, which was situation 2 = paid GST for gold cards, upgrade to Plat later. then no new GST charged to Plat (since i've already paid GST earlier).