I've never transplanted one that wasn't root bound before, but my 3 gal smart pots have the same circumference as my 5 gal, just not as deep.
When I transplant I let it dry completely out(happens fast when they get bound) then pull the edges of the pot away while working my hands slowly around the mound to free it from the pot. Then once it's out I take a blade and slice top to bottom on 4 sides and loosen the roots up with my hand that isn't holding the base of the stem, not roughly ripping, just some gentle tickleing and shaking to narrow the mass up a bit so it will fit in its new home, I pre fill the new pot with a few inches of medium and water it, then drop in the plant, add more coco and pearlite mix to fill to top, then water, done.