Hi,
As a frequent user of Eurotunnel, I'd just like to confirm that it's as easy and flexible as others have said, but also a bit more complex too...
The more complex part is that the crossings are divided into different price bands - if your new earlier (or later) crossing are in the same price band - you should be offered a crossing (at no extra charge) providing there is space...
If the earlier crossing falls into a higher priced band, then you will be offered the earlier crossing - for the price difference...
Robert