@Gaius5 The question is "why do they want to cancel?' Is it due to them being unhappy with the accommodation or because their plans changed?
You mention "issues" so for the sake of this post I will assume they had problems with the accommodation in some way shape or form. It really depends how big these issues were and what you did to address them. For example, if they said they house was not clean and you sent a cleaner over, this would be appropriate action and response. If they said there were bugs and you sent an exterminator over, again appropriate action and response. If they are saying the beds are uncomfortable, then that's more subjective and something you really can't change.
Obviously if you allow the change, you will be refunding them. Without more information it's difficult to give you a straightforward answer. There are a few things you can do
-If you feel the issues, they brought forth were addressed properly, then I would offer them a refund based on the ability to rebook their dates.
-If you feel the complaints were valid and ongoing, and/or something you couldn't fully address, then I would consider at least a partial refund
-You can refuse the change and not offer a refund if you feel they are requesting with ill or untrue intent