this was a letter I did over 10 years ago when I swapped over, I lost 2 customers one of which came back to me.
I did a similar letter. We purchased a trad, glass cleaned only, round. WFP was a new venture, and I was scared of losing these customers son had paid good money for. (The round was on 1 estate and consists of mainly dormers. My son purchased it without me knowing, and I wasn't going to walk all over roofs to reach dormer windows.) WFP was the only solution and was new 18 years ago. Few knew anything about deionized water. Some didn't even know what distilled water was. (I often questioned the British education system, tbh.) The only thing most understood was that rain water on clean glass didn't leave spots. We were also new cleaners to the round, foreigners with a new cleaning system nobody has seen before. Yorkshire folk aren't very trusting either; I know, I married one.
The first day was a nightmare as those frames just ran dirt and some windows had to be done 3 times. But the results were remarkable, and even I was impressed with the first day's results. News quickly spread on the estate. This did a lot for my confidence in WFP. All our new customers wanted their windows cleaned with this new system. In the end, I never needed to hand out many of those letters.
We also lost a couple, but we gained many more because these home owners didn't want window cleaners walking over their roof to reach the top windows and breaking tiles.
But here is the kicker for all those changing their rounds over from trad to WFP. This round was based on a 2 weekly clean. Very quickly we started to get; "can you leave it this week as the windows are still clean" excuse. If your round is based on a 2 weekly schedule, for example, and they want to change it, then you need to renegotiate the price per clean at that moment. From experience, they ended up wanting a 6 weekly clean for the 2 weekly price.