Hi Lee,
Personally I would just do it WFP and be done with it.
Most people are aware of the pole system now, so any explanation can be done as you do it.
I think it would be more important for you to get the window cleaner you are buying the round from to introduce you to his customers. Also, try not to use the phase of buying the business or their custom, but rather that you will be taking the round over from ..............(windies name)
We took on a Trad round 6 years ago, window frames minging and windows cleaned to a poor state very irregularily.
It took us ages to do all these first cleans but the results spoke for themselves and we hardly lost any. At one stage I was doing 3 or 4 houses a day, but only left when the windows were streek free - none of this "it will take 3 or 4 cleans before they are perfect" spiel.
I was also new to WFP as we bought the system to clean these houses as most were dormers. So this was first time out as well with Ali Unger poles and a heavy Vikan brush on the end.
You have been WFP ages and will have the confidence to get on with it. Even if they are in when you clean, I would move onto the next house to give the last one time to dry off. Check them and then knock for your money.
As we have got more experience now, we very really have any issues with new cleans, other than it takes much longer to do them first time. The same will apply to you.
Spruce