There seems to be precious little info on it on the web which I suppose we could take to mean it may have been happening for a long time but the Turks maybe still had no writing etc etc.

http://www.fordham.edu/halsall/med/saunders.html

I found this link above googling around, the only other link I found is this one fromIslamWatch but I dont know how credible a organisation like that would be

I have a book a history on the arab conquests and while it details the various campaigns it does not really delve into WHY the Turks converted unlike how it delves into former Christian Syrians etc doing the same.