The torque converter is a welded assembly and the lock-up clutch is captive inside this welded assembly.
LUC piston shown in green, friction plate in red
Piston
Friction plate
The torque converter can be replaced without opening up the transmission but the transmission assembly has to be removed from the vehicle to provide access. The torque converter simply slides off its splines and no specialist transmission knowledge is required to replace it – any competent mechanic should be able to manage it. The only tricky bit is ensuring that the oil pump drive dogs are engaged when it is replaced.
The lock-up clutch is fully engaged in the higher gears only to reduce the slip across the torque converter (to save fuel). It is not engaged in 1st or Reverse so I can’t see why it would contribute to a clunk on gear selection.
The LUC is either fully on (locked solid), off (disengaged) or in its ‘continuous slip’ mode (approximately 3% slip) which is used to eliminate torsional vibration at lower engine speeds as ZF decided not to fit a damper to the Sachs W260/280S-2GWK unit to save weight/space.
Phil