What precautions to take when moving a mirrored server
I would remove the witness from the session. The reason being that if the 1 online partner (principal) loses communication with the witness, the principal will take itself offline because it has no way to know for sure that the mirror isn't online.

There is no need to change the operating mode. When the principal becomes disconnected, it operates asynchronously. When the mirror reconnects, it will continue to operate asynchronously until they are synchronized, and then it will return automatically to operating synchronously.