In this sort of situation I would expect most hotels to be understanding and switch the booking for free - provided that rooms are still being sold at that price on both dates; or one contacts the hotel immediately after booking and noticing the mistake.

I have never understood the origin of that saying. What is the purpose of "having" a cake other than to eat it? It will grow mouldy or stale.

There are somewhat unethical things one can do to mitigate the risk of making mistakes, such as using fake CC numbers etc. Also, if booking a non-refundable reservation direct with IHG, if the hotel has not yet charged the card provided, you can change the reservation online to a different date and in most cases the hotel won't notice - if they do notice and decide to charge both reservations then you haven't lost anything beyond what you already thought you would lose.