You’re talking about cats. Your friend suddenly starts talking about dinosaurs. Is that a whopping topic change?
Yes
You’re talking about your pet cat when your friend asks what cat food your cat eats. Is that a whopping topic change?
No! You are still talking about your cat.
You’re talking about going to the movies, and your friend starts talking about getting popcorn. Is that a whopping topic change?
No. Movies and popcorn go together.