You can change it going forward though. It's really hard to change a contractual term like this for existing client's because that's not the agreement they came under. But if all your current client's have already been with you over a year, then you could make the change immediately, issue the revised policy and reassure them that it won't negatively affect them having already completed the required term.
Otherwise, just change your master documents and anyone new coming in, comes under the new terms. Over time, those you have now who aren't under the new terms, will pass the one year anniversary or move on, leaving their replacement to accept the new terms.