How is that not a violation of fiduciary standards (not to mention the possibility that what the sponsor wants to do is for the sponsor's benefit and not that of the participants)? Certainly, it would seem to be hard (if not impossible) to justify charging larger fees to terminated participants than to ongoing active participants. You only get to charge participants reasonable and necessary expenses. How would this be necessary?
How big is the plan? Is this related to trying to hold the number of participants down to avoid needing an auditor's report? Even if it is, I don't think you get to discriminate against terminated participants to force them out.