How to buy Beijing subway tickets if you are changing lines?

You just need to purchase to the destination directly.

I think the station selection is just to help you get the right fare which is based on how far you are going.. but the ticket then covers you for going to any station that would cost that amount.. and you can change lines as much as you need as long as you don't exit the station barriers (coz then it keeps your ticket).
In Beijing you can pay with 1CNY coins or 5 or 10 notes.