When you became a yearly diamond member, you paid for a year (that's what the yearly means -- it's cheaper than monthly because it's for a whole year, can't just cancel it after a month).
Now when you upgrade your membership, the value of the current membership is deducted from the price. So I would guess that in this case, the value deducted was larger than what you had to pay, so you paid nothing extra for the platinum membership. But that is just a guess.
But in that case, you did pay for diamond, and only have the features of platinum now.
If I were you, I'd send a mail to staff, to see if they want to restore your platinum membership.

Hi,
about 5 days back, I became a diamond yearly member, but yesterday, I decided to change to yearly platinum membership...So, i did. Does that mean I will lose the money needed to pay for a yearly diamond membership ($95) when I only used the diamond membership for a few days? Do I have to pay for both the diamond and platnum memberships now? Can someone please explain the concept.
Worried,
dragon27