となると / となれば

となると takes a situation as settled – something you have just learned, or a case you are considering – and says what follows from it: "if that's the case", "if it comes to". It goes after the plain form of a verb or adjective, or after a noun with or without だ. となれば means the same.

The second half is usually a consequence, a worry or a judgment. Often the premise is news to the speaker, and となると is them working out what it means.

"When it comes to"

After a noun that names a topic, となると means "when it comes to", and the second half says how someone changes on that subject.

となると, となれば or たら

The two mean the same, but となれば leans towards resolve: "if it really comes to that, then…". It suits a second half that states what you will do.

たら or なら simply states a condition. となると treats the condition as already decided or found out, and draws a conclusion from it. In casual speech it becomes となったら.