its hard at first as you will just have to accept it unless you try get a bit extra cash out of them for changing to bi monthly but then you run the risk of them saying 'o i think we will leave it then thanks'
i think a fair few of my customers would change to bi monthly,but im not going to tell them,if they do ask i would just charge 50% extra,plus if your charging high prices you proberly would get more asking to change frequency