I would allow the switch to Friday and charge for 3 days if I had a good relationship and this was a good client. I'm flexible with my really good clients who never try to push me but if it's somebody who is always bothering me in small ways by pushing their limits, then no, I wouldn't do it. If you do it make sure you stress that it is a one time exception because of the sickness and you don't normally allow switching or they will start doing it all the time. You can count on that!