The only way to switch them back is to have sex with one of them until you both orgasm.
They have to be lucid and awake. You have to be lucid and sober. Everyone will remember the experiences after they switch back. If they remain in the switched bodies for one week they both die a painful death.
Are you fucking your Significant other in your parents body, or your parent in your significant others body?
This is a famous one.
Both!