If it is due to medical reason then no, they don't move on to another career. Mainly due to the distraction that the medical condition might cause for the dog, so the dog can't focus on its task. Also, it is likely that Daisy at some point might have to have surgery which means she will be out of "service" for quite a while. This makes it difficult for them to get back into it.
Non medical reasons for withdrawal from the program would be things like Anxiety, Distraction and even Aggression. All of these makes the dog unsuitable for service. It's better to let them go to a loving home where they can get the love and attention they need.
That is very interesting. Thank you for answering my questions. :)