@Matteus0
Actually, it is not quite as straightforward as that, as it does depend on the length of stay (is it long term, i.e. 28 days or not) and the notice the guest gives you for the change.
For example, today I got a date change for a long term guest who is coming in over 30 days time and meant that she shortened her stay. Because it is more than 30 days away, I was not asked to accept or decline the change, rather it was automatically applied.
So, it is always best to double check the cancellation policy that applies to that particular booking. In some circumstances, you have the option to accept or decline and, in others, you do not.