Not quite. There's nothing special about the Excursionist leg when it comes to the way inventory is handled. What you may be thinking of is that all downline partner segments must have current inventory for any change, including a change to the Excursionist Perk.
(I'll use DEN-YYY to refer to your existing flight and DEN-YYY' to refer to your desired flight).
You can't change just one of them, because they're not ticketed separately. The DEN-YYY' inventory doesn't matter in the slightest. The only thing that matters is the XXX-(DEN)-YYY' inventory, which can be managed separately from XXX-DEN and DEN-YYY'. The same would be true for a cash ticket, BTW -- if you were booked on a K fare from XXX-(DEN)-YYY, and saw K inventory on the earlier DEN-YYY' flight, you still wouldn't be able to switch to it unless XXX-(DEN)-YYY' also had K inventory.
It wouldn't be any different if this were a one-way award or your outbound or return flight -- you have to look at the inventory from origin to destination, not the inventory on the individual legs.
In other words, yes, that's how it works, and no, there's not likely to be a workaround to try to get it to price the way you want it.
