I see the adjustment, that certainly makes more sense in mathematical terms, giving exactly the same 50% (equivalent) discount to everyone.
In marketing terms, however, I think you are still missing an opportunity by not targeting existing Gold and Platinum members more aggressively - these are guys who have already established that they are willing to pay money but who, for whatever reasons, chose NOT to come in at the Exclusive level ... why?
Presumably, they gave it some consideration and finally decided that, in their circumstances, the benefit of Exclusive over Platinum was not worth the additional cost. If they have been members for a while and have remained happy at Gold or Platinum, there is not much chance that they will suddenly change their minds and decide to pay more.
In those circumstances, the only way to shake money out of those particular pockets, and to increase the overall revenue generated by your site, is to run an offer that WILL strike them as worthwhile. Obviously, it's not my site, you are the experts on your own business, I just have a feeling that you might be leaving money on the table by not recognizing how the value proposition differs between new and longtime paying members. Finding a way to lure, say, 20% of your existing Platinum members into becoming Exclusive members would also increase participation by those members and increase the visibility of Exclusive memberships, thereby attracting more newbies to that membership level in the future when you are charging full price again.