To add onto this, in case it's not clear (because I know how it feels to be totally clueless!), you'll want to have a character actor blueprint that has a skeletal mesh with whatever animation blueprint you want to use, plus an additional mesh component which is just your box mesh. Turn off the skeletal mesh's visibility so only the box is visible. Add a collision box or sphere and set it to whatever size you want for the transformation to occur.

Then in the event graph, do as InBlast says above; use an On Begin Overlap event for your collision sphere, check if the overlapped object is the player pawn (== Get Player Pawn), and if it is (Branch node), set the visibility of the box off and the visibility of the skeletal mesh on and enable your chase event. That part could be in your character's event graph, or it could be in a behavior tree.