Does frogging and starting over something that has been on the needles for a while have any effect on the yarn?
Other than making your yarn all squiggly, no. Well, I say that. If your yarn is very fine then you MIGHT encounter it tearing/breaking but 99% you just frog it and give it another try. Only damage done is to your sanity. 
Or as I like to say, your pending insanity. 
