You have a TTK Legendary Chest piece with 240 Defense and like the stats and perks on it.
You later get a 260 Defense TTK Legendary Chest piece but don't like its perks or the way it looks.
Solution: you inspect your original, 240 Defense Chest. At the far right you'll see an Infuse option which costs 3 Legendary Marks, a Mote of Light and some Glimmer. Once selected, it will prompt you to pick which item you'll infuse. Note that it has to be the same class, and it must have higher defense than the one you're upgrading. You select to sacrifice the ugly 260
Your old chest piece will have consumed the new one and emerge at 258 Defense. Very close, but not at the same level, as the sacrificed piece.
Works the same for weapons.