I think what they're saying -- which hasn't been explicitly explained -- is that if you change the return flight now before travel has started the whole ticket is repriced using prices currently in effect as if you were buying the ticket today. If the travel is soon that would mean paying "last minute" prices. However if travel has begun then it's repriced using "historical" prices which means it's as if you were buying the new fare when you bought the original fare. So it's eligible for the advance purchase fares that require a certain number of days between purchase and flight. In a case like this, assuming the original fare allowed an open jaw at all (it probably does but you never know) and assuming that fare class is still available on that flight (that may or may not be true though) it could mean you get the same price for the outbound fare as you originally paid.

But looking up at the original post the flight is in September.... it may well be worth calling now. That's far enough in advance that the same fares may well be available.