I too was very concerned about switching and at first did the letter thing and explained it to all my customers. In return a lot of them said, as long as my windows are clean, dont care what you use! Think my speal was too long n boring lol!
So in the end i just carried my tds stick with me. For the old dears to get their heads round it, asked them for a glass of their water and show difference. Thats all it took in the end. It still took maybe 2 to 3 cleans before it sunk in that YES the water is left on the windows, lol.