Question about Nancy Drew books?

Hi! Okay, so I know the original 38 Nancy Drew books came out, but then they were reprinted up to a 64th book later on. Can someone please explain this to me? I have 2 copies of the same book, one from each set and there is a major difference in the wording. I think I read somewhere that they were just rewritten to be more modern? Can someone just explain to me why this was done? And why there are still about 30 more books after they rewrote them? Thanks so much for any help!