This question bugged me quite a lot and I think I may have finally found the relevant quotes that may answer to the limit its possible. Emphasis mine.
TOR Questions of the Week, February 2005-July 2005
Week 15 Question: When a channeler is forcibly turned to the Dark, is his/her former personality lost to eternity? Are they in a permanent state of mindless Compulsion? Furthermore, can a channeler forcibly turned to the Dark return to the Light unaided?
<PERSON> Answers: They are not in a mindless state of Compulsion. Their former personality is twisted, the darker elements that everyone has to some degree elevated while what might be called the good elements are largely suppressed. I don't mean things like courage, which is useful even to villains, but they are unlikely to be very charitable, for example, and forget any altruistic impulses. Call it being turned into a mirror image of yourself in many ways. It is very unlikely that a channeler forcibly turned to the Shadow could find a way back to the Light unaided. 624
For one reason, by virtue of the twisting he or she had undergone, it is very unlikely that he or she would have any desire to do so.
Towers of Midnight book tour 16 November 2010 WH Smith, Paris, France - <PERSON> reporting
Q: Can people who were forced to become Darkfriends through a 13-13 circle be saved?
<PERSON>: He said he couldn't give an answer, but that the characters from the books would say yes.
Both <PERSON> and <PERSON> seem to suggest that the Channelers once turned can be "saved". <PERSON>'s answer seems to suggest that with some aid from others they can be turned back to light. While <PERSON> gave his usual answer for the Wheel of time books.
My personal conclusion for this is that with some help from the people that shared a connection with the "turned" Channelers, they can be saved/turned back to light. However, they will never be the "same". I think they will stay in the same state as that of <PERSON>, i.e. more colder, harsh and distant, but essentially for good. 773