No, it cannot be applied retroactively: this is essentially a contract law question, even though it arises in the context of LLC ownership, since it goes to the agreement(s) governing membership and the rights or benefits thereof. Contracts or agreements cannot be made retroactive in this way; people cannot be held to additional obligations or face additional penalties that did not exist at the time of their actions. If they try to apply it retractively, you would seem to have good grounds to sue them to overturn that action. Going forward, of course, the change is completely legal.