I found this in a an article on WDW News Today:
"any guest who uses a FastPass under tickets not used for that day’s theme park admission will have their associated MyDisneyExperience account and the ability to book any further FastPasses locked at the end of that day. The system will apparently run through all accounts to find any culprits overnight, and then lock the account should it find a FastPass was booked illegitimately. Guests with these infractions will have to visit guest relations in order to unlock their accounts. Guest relations will only unlock the accounts if the act was deemed to not be malicious."
Ok, so, if Mom and sick Billy stay back at the resort (DO NOT use their admission to enter the park) and Dad and Sue go to the park, Dad and Sue cannot change the party of Mom and Billy's FP to Dad and Sue and use them because Mom and Billy's fastpasses were booked under tickets NOT used for park admission that day. (Finally have an answer to that!)
What if Mom and Billy DID go to the park but left early and Dad and Sue are still there. It seems that they can "legally" change the party of Mom and Billy's FP to Dad and Sue because Mom and Billy's tickets WERE used for admission that day.
What I'm reading doesn't say anything about the tickets having to be in the park at the time the fastpasses are transferred to someone else or at the time the fastpasses are used.