Front Sprocket

Another thread was discussing front sprockets so I went to look at mine.



How much wear has to happen before replacing it? The chain and both sprocket (installed as a set) have about 15K miles on them but only the front sprocket looks really worn. The rear sprocket looks almost unworn / new and when I try to pull the chain off it per instructions in the manual (to test for wear) I am barely able to lift it at all.



Would just changing only the front sprocket destroy the chain? Personally I think not and I already have a very slightly used front sprocket to swap in there but I was wondering what others think and what their experiences have been. (And should it all fall apart I would buy two new sprockets and install my new sitting on the shelf o-ring chain. I have to add I am not planning on swapping the sprockets anytime soon.)