Not sure about how serious the tournament.
I mean… If you read my story on the old post i mention we had tournament between company yearly (its team format). Its more on serious & strict rule. Register the player 3 double or 5 double or 2 double 1 single + 1 or 2 backup player (decided by the commite that year) & if on the day of match someone not coming, we could replace it with backup player & if there is 3 people is missing, we automatically lose 1 match as we are not allowed to have the 3rd backup player. Another limitation is that when the person is played, then its locked & cant be replaced till final match. So let say in 5 group format. Till before QF i always win 3 1st group, then i had the chance to modify the 4th & 5th player. But if i had a hard match on 1st round end up playing all 5 group then im in abig trouble if next day someone not coming.
Other hand we also had fun tournament sometimes (not always) for internal company where everyone can join (i mean even a security or a cleaner are allowed to join). Its less strict rule & more about fun. When someone missing a partner, everyone is ok for him to find other partner as long the player didnt play with different person. But ofcourse the level is at best high beginner here & the only complaint few year back is someone dont have a partner, he saw me (as a spectator actually) & ask me to be his pair. But ofcourse everyone complain as they all know im part of the main squad of the company
So i guest its something to be discussed or decided upfront so everyone wont make a big fuss on the games day.