Why dont you redo your contracts for all new clients NOW, and send out a notice for 2013 for existing clients. That gives then the rest of the year to use their promised vacation time and such. After that.....the rules of the game change. I usually give a month or two notice when making certain changes i.e. rates or hours change. But for little stiff...I change immediately. If I were to change my vacation time, I think it would be very fair to let them have this year as promised and start new in the New Year.