S.A.
we kept the old bottles/nipples for our new baby. I don't it that was the right or wrong thing to do though. The bottles we used were Playtex Drop-Ins, so the liners were disposables, and we hand-washed the nipples the first time around.
I DO know that plastics break down over time, especially if they are exposed to high heat dishwashers, microwaves, etc. So perhaps it is best to get a new set if the old bottles were washed in the dishwasher and/or put in the microwave. But this is just a guess on my part.