i would treat him essentially like a new client and i would give them a quote like new and have 3 different frequencies on it
include 4 weekly, 8 weekly, and quarterly
quarterly cleans are one-off cleans each time so price accordingly. for me that would be double.
i would also have a minimum number of cleans at each price and add that they get charged for the full term of cleans if they cancle before this time
it's up to them to choose the one that suits his business best, your price has to suit your business too
with this approach, you'll sometimes find they decide to stick with what they have got already. he was probably thinking initially it would be the same price whatever frequency.