My official policy is you cannot switch days - typically because I have always been full this has never been an issue ... my rationale is full time clients are providing the most consistent reliable income and they cannot make up days if sick so why should part time clients who are already a financial risk if you cannot fill their opposite space in comparison be given special consideration to make up or switch without payment for the extra day???

I know that is cold business logic and I admit that for the right client who did not expect it of me I too would likely make an exception of allowing them to pick up the extra day at not cost if I had the space.