A very very small minority of customers will not take to wfp come what may, but most will go on the results.
The problem with converting to wfp is that most wfp retailers will tell you that you can go up to twice as fast as trad, maybe possible if you've only been cleaning windows for a year trad but if your a long standing trad cleaner then to go twice as fast is gonna be real hard. To make it worse converting windows to wfp would normally take up to twice as long as trad, but now the new inexperienced wfper is going to try to do the job in about a 1/4 of the time he should as he thinks he should be finishing by lunch time. The results are normally a horror story.
If your that worried stay on a ladder, however if you make the change, do every window twice (frames and glass and then just glass). Maybe convert the ups to wfp first and leave the downs trad for a while until the custys get used to it. Tell every customer to call you asap if the windows come up bad and that you will redo them. Before long all your custys will be amazed at how good this new invention is.
As regards shops and wfp. I personally wouldn't but I once worked with a guy in Grantham who does most of the shops in the town wfp, even double handed wouldn't beat the speed he went and he did a good job. He had static hose reels though which makes a big difference. He did loose one job from wfp though.
Hope it goes well. Simon.