Chained payments in Marketpress: Can the percentage be changed for future members but still be kept

I am using chained payments with a network of stores in multisite and taking a certain percentage of the sub store owner’s profits for the mainsite. As an incentive to get users to join as initial members, the percentage is low and I want to keep it low forever for these initial members. I would want to increase the percentage for future members after this initial introductory period but still leave the old percentage for all the original members. Is it possible and how do I do it?