Haven't read all of the replys, so sorry if I repeat someone else's thoughts.
First, the context of Proverbs 4:18 ... if you read the complete context, it is NOTHING about truth changing and getting more and more perfect.
Second, where does the new light come from? If it does indeed come from Jehovah, how is that different than being inspired? My Websters dictionary says inspiration is to communicate ideas from a supernatura source. If not from Jehovah, then from whom? If from Jehovah, it must be "inspired", yet they claim they are not inspired, otherwise, there would be no need of changes.
Third, why do we ask others to check what they believe, yet are so unwilling to really check deeply what JW's believe? It's ok to question, if "sincere" before one's baptism, but not ok after!
Just a few thoughts, hope they help.
Peep