Well, here I am again, Mr Karl who is still not cleaning but seems to have something to say on most things

pylofm, I think I would do a few calculations before I went about changing my business model.
Something worth considering is calling any old failed estimates and offering them the 8 weekly clean at a slightly elevated price.
The next thing I would do is to use it as business retention tool. If someone no longer requires my services I would offer it too them with the "are you aware that I also do an 8 weekly clean that works out much cheaper".
Then, I would test the market. I know you use flyers. I would send out 500 in one area, 500 in another and 500 in a different one. 3 areas. I would then quote these areas seperatly, 4weekly, 6 weekly and 8 weekly cleans. What I would be trying to do is find out if I had a different take up rate on my quotes. Give them the one price unless they ask. The price for the area and the frequency.
All you are doing is testing the market, it is a very good idea and it is something everyone should do before changing their business model no matter if it is finding out if they could get more customers with WFP or pricing and frequency.
I would do that anyway, if anything it will be an interesting experiment for you and will give you some idea if it is worth doing.