i will inform the customer that if they are not happy after 3 cleans then I will go back to the old method
I would not tell them that you would go back if they dont like it as you will always get a few who will say they don't like it.
I changed all my customers over to WFP in one go and told them that i am no longer using ladders only for access.
and i only lost 1 customer a few did say i could do that with a hose or something and i told them to go ahead as they wont get them clean.
People will take to it no problem and a lot of people have already seen some one using a WFP.
you will pick up a lot more customers than you lose.
I did one house traditional a couple of months ago as i ran a bit low on water and she moaned saying she likes the new way of cleaning and wants that in future.
Paul