Can I swap my Dragon in the middle of a battle for another one?
Yes, you can!
You can swap your dragon when it's your turn. Without losing a turn!
You are also shown in the battle screen what dragons you have left on the bench and their life. If you tap on the Dragon portraits shown on the screen, you'll see a bit more information about the Dragons.
This can be really useful to avoid Dragons being "Tired" after battle.