Flip the couch upside-down so its legs face upward. Remove the thin, wooden legs to be replaced by rotating them counterclockwise by hand. If you're unable to twist the legs off, clamp the legs with locking pliers to get a firm attachment to them and provide the required torque. Set all of the legs aside.
Measure the diameter of the leg bolts. Purchase couch legs that have the same diameter and install them by rotating them clockwise into the holes.
Make couch legs, if desired, by drilling a hole with a smaller diameter than the bolt in one end of a 1-inch-by-1-inch-by-6-inch block of wood. Remove the bolts from the old legs by attaching locking pliers to the bolt and another pair of locking pliers to the couch leg and twisting them opposite directions.
Screw the removed bolt into the hole in the end of the new couch leg by clamping both the bolt and the new leg with locking pliers and rotating the bolt clockwise while holding the leg in place. Install the legs in the couch by rotating them clockwise in the holes.
Use new bolts with the couch legs, if desired, by holding the new bolt without heads against the hole in the couch legs. Grip both parts with locking pliers and rotate them in opposite directions to install the bolt. Install the couch legs on the couch by setting them in the holes and rotating them clockwise. Flip your couch back upright and ensure that it is level.