Came across this quote from w64 2/15 p.101:
"When a belief is not correct but is wrong, then it cannot be right at the same time. The most sincere belief and practice of that belief will not change it from being wrong into being right. Nor will it become acceptable to God as truth, since he does not take pleasure in falsehood."
I honestly can't argue with The Watchtower here! However, this quote might help the thinking JW (few that they are) see that "old light" really is falsehood. When WT teaches something that gets revised, like the recent change in the FDS for example, then it means that for the many decades the "old view" was taught, WT was really teaching falsehood. And... as the quote concludes: God "does not take pleasure in falsehood."
Old Light = Darkness. No matter how sincere a JW's belief is, he was teaching a falsehood. That makes him/her a false teacher.
Reminds me of something the apostle Peter said, "However, there also came to be false prophets among the people, as there will also be false teachers among you. These very ones will quietly bring in destructive sects and will disown even the owner that bought them, bringing speedy destruction upon themselves." -2 Peter 2:1.