I agree that you can amend a plan to discontinue participation (but stand by my statement - with emphasis added - "you can't retroactively change eligibility to remove someone that way.")
I get a little - ok, totally - bogged down in the technicality of saying you retroactively amend as stated, and that that means that the person was in from 1/1 to 12/16 and then out upon signing of the amendment. I think you have, in fact, retroactively excluded him back to 1/1 or 7/1 or whatever date he entered. I think the signing date is a red herring; if the amendment says it is retroactive, then it is retroactive.
I think, at least, you'd have to clarify the amendment to say "but if someone entered prior to the signing date of this amendment and would not have entered under the terms of the plan as amended, they are excluded as of the signing date." Or something like that. Maybe counting angels on the head of a pin.