I don't know that she would want to, but hypothetically under the rules, could Serena find a new partner to fill the spot because play has not begun, or does the team as a whole have to retire, giving the spot to the next team on the qualifying list (if that even exists in mixed)?
I wondered about this too. I assumed that because there is a deadline for submitting your team, there would be no changes allowed after the deadline. Could be wrong though. Edit: Also, if you could change partners after the draw came up, it would allow people to game things a bit?