It's your business you can change the rates if you want to!!!! I think that is such a fair amount of time to let them prepare. When I change my contracts I just call it an addendum an add to the previous contract. After 7 years I changed my rates when I realized I had been charging the same going rates from 7 years prior. None of my parents had an issue, (sure they went home and let out a little sigh! :0) but they completely understood and said I was worth it