I would think they'll treat the change as subject to the normal change fee ($150 + fare difference), though don't know what impact Priceline will have (could be theirs don't allow for any changes at all, even with a fee--I just don't know).

But typically, IME, UA doesn't allow free changes due to illness; otherwise, people would be trying to get out of the change fee on nonrefundable tickets all the time by claiming they were sick.