Rachel: yes that is exactly what I do. during the year I make changes to the master and then everyone gets a new copy at the end of the year. during the year though if someone new starts then they get the updated contract as it is at the time so potentially different rules for everyone.
That is how I understood the original post as well.
flowerchild: don't worry about it. just adjust it next year saying that you will be renewing your contract every January and that any major changes will come into effect 1 month (or whatever you choose) after you give your new contract. If they don't like your new policies they still have enough time to give you proper notice that they are leaving.