Rules FAQ
You can play this on a participating Knight character who is already standing to remove it from the challenge. However, it must both stand and be removed from the challenge for you to kneel another Knight character to participate.
Action: Choose a participating Knight character you control. Stand it and remove it from the challenge. Then, you may kneel another Knight character you control to have it participate in the challenge on your side.
Link: Decklists
Rules FAQ
You can play this on a participating Knight character who is already standing to remove it from the challenge. However, it must both stand and be removed from the challenge for you to kneel another Knight character to participate.