Egg Event just for new eggs?

Unless something strange has changed, eggs have always been set at the time that you receive them.

The species inside the egg is determined when you get it from the stop, not when it is hatched. In other words, only eggs obtained during the event will hatch 5/10 km species and can do so even after the event.

But I believe that ANY egg you hatch during the event will give extra candy though...

The species is determined when you get the egg. The amount of candy and stardust are determined when you hatch.