I want to start getting some characters to level 20 before I become a member, so I was going to play the free version for a while. But, can I use the characters I created in the free version when I become a member?
Yes. They do in fact carry over. If your subscription lapses your account reverts to a starter account, and you can play characters at or below level 20. Here's some more official information: https://us.battle.net/support/en/article/world-of-warcraft-starter-edition