I think the problem is the tying of "number of coins" image with donator and with post count - it's already bad enough that a "newbie" can donate 10 btc and suddenly look like a "hero" but with this idea people with hero post counts (not that I think large post counts are at all something that really counts) suddenly will look like "newbies" unless they fork out more cash.
Perhaps would indeed give the forum more btc in donations as all those donators rush to get back their 5 coins.
I am assuming you are going to donate enough to have 4 1/2 green coins (from your own list you won't be able to keep your current 5 coins).
Well I was implying, 1/2 green:4 & 1/2 orange coins,
1 green & 4 orange coins, etc.
It still be five but compared to a ratio of green/orange etc.
I am thinking that 50 BTC= 5 green coins, and then 55 BTC=1 purple coin, raising all that by 5.
Might be a better idea.