The AoS designers commentary says this:
Q: Some abilities say that a unit fights at the start of the combat phase. What happens if that unit is not within 3" of the enemy, but later in the phase an enemy unit piles in to within 3" of it?
A: A unit that can fight at the start of the combat phase but does not do so is allowed to fight normally during the combat phase should an enemy unit move to within 3" of it.
Then again, the Commentary also includes this pile of ox manure, so they may be talking out of their donkeys in general.
Q: Let’s say Unit A has been made to fight at the end of the combat phase. If, when Unit A fights at the end of the combat phase, its pile-in move results in Unit B being eligible to fight when previously it wasn’t, because Unit B was outside 3", can Unit B now pile in and attack?
A: If Unit A is from the army of the player whose turn is taking place, then Unit B will be able to fight (as its opportunity to fight comes after that of Unit A, and it will be eligible to fight because it is within 3" of the enemy). If, on the other hand, Unit A is part of the army belonging to the player whose turn is not taking place, then Unit B will not be able to fight (as its opportunity to fight came before that of Unit A, when it was not within 3" of the enemy).
So in one case it's ok for your unit to fight later even though "it's moment" has passed, but in the other case the opportunity to fight is lost altogether, just to conform to their made-up "I fight last, you fight last" policy.
