AP attacks with Duskdale Wurm (a 7/7 trample creature). NAP blocks with Quilled Slagwurm (an 8/8 vanila creature). AP casts and resolves Kor Chant, choosing to have all damage that would be dealt this turn to Duskdale Wurm by Quilled Slagwurm to be dealt to Quilled Slagwurm instead. When they move to the Combat Damage step, can AP assign trample damage to NAP?

CR 702.19b The controller of an attacking creature with trample first assigns damage to the creature(s) blocking it. Once all those blocking creatures are assigned lethal damage, any remaining damage is assigned as its controller chooses among those blocking creatures and the player or planeswalker the creature is attacking. When checking for assigned lethal damage, take into account damage already marked on the creature and damage from other creatures that's being assigned during the same combat damage step, but not any abilities or effects that might change the amount of damage that's actually dealt. The attacking creature's controller need not assign lethal damage to all those blocking creatures but in that case can't assign any damage to the player or planeswalker it's attacking.

We have some discussion about the interpretation of the bolded part of this rule, because of:

CR 510. Combat Damage Step; 510.1. First, the active player announces how each attacking creature assigns its combat damage, then the defending player announces how each blocking creature assigns its combat damage.

Looking forward to an explanation that will satisfy the Rules Gurus at my local discussion forum, thanks!

Greetz, Dustin.

In this scenario, the active player cannot assign trample damage to the nonactive player. Trample does not take Kor Chant into effect when it determines how much damage needs to be assigned to the blocking creature for the same reason it doesn't take damage prevention effects into account, even if the damage that will be added by Chant's effect will also be combat damage that was assigned at the same time.

The relevant part of 702.19b is actually the one immediately following the part you bolded: “…but not any abilities or effects that might change the amount of damage that's actually dealt.” Redirection effects such as Kor Chant are exactly that: an effect that might change the amount of damage being dealt to the blocking creature.