As you know, 75% of your credits just "disappear" when you buy an ad spot. Let's say someone buys your ad slot for 64 credits. 48 of those credits just vanish--you get the remaining 16. EC does this to control "credit inflation."
Maybe they won't have to do this after the system changes. I like the idea of keeping more credits, and I think it could happen. Let's say only 50% of the credits disappear--or what if none of them disappear?
I'm not sure this will happen, but it is a possibility.