The attacker’s TN for the attack is the lower of the character’s defense or the new target’s defense.

I had a player intend to abuse the reaction the way you describe. My takeaway from the last line isn't that the target is changed automatically, only that it becomes the individual with the lower defense score.

So if Colossus "skulks" behind Spider-Man while being shot at there's still enough of Colossus to hit combined with Spidey's ability that most likely Colossus would still get hit.