Ok, here is what i'm trying to do. I'm trying to make an escort that asks to be taken to the Chaos shrine. Once the escort is there, I want to change his body to that of a demon (modify stats a bit too maybe) and then have him attack the player. I'm pretty sure I know how to get the escort done, but how can I tell him to change forms (set the body property) when the destination is reached instead of completing the quest?

I would do that using a triggered spawner placed at the shrine that detects the player and/or the escort and then just applies the changes to the body value and any other stats you want.
You can use the SETONNEARBY keyword to refer to the escort.

I wouldnt use a standard ESCORT objective since you really dont want the objective to be automatically completed by having the escort reach the target region.
Just use a custom objective with the triggered spawner at the shrine.

how would you get the NPC to follow them out there then?

you could use an escortable npc, just dont use a standard ESCORT objective for the quest since you arent interested in completing the objective by having them arrive at their destination.

To abort the escort objective before it autocompletes on arrival, just set the Controlled property on the npc to false before they reach the escort destination. That will cancel the escort.

You could also just delete the npc and spawn a new daemon in his place when they arrive.

Thanks, i'll work that in.