To swap the lines, just take off the old lines with a pan underneath and swap them out. No need to drain the whole trans. You won't leak excessively while you are doing it. But it will leak a little.
Once the new lines are on and you done, fire it up, check the fluid level and top it off if needed.
IIRC you need a 16mm wrench at the radiator and a 16mm and if they fight an 18/19mm as well at the trans.
When the fittings at either end don't seem to want to thread. Check the angle and keep on trying gently. Once you get the right angle, they'll go right in. Many first timers can get frustrated at how they can be a pain to align.
