Conway went off script in that one adding some extras and caught Korman by surprise making him wet his pants. Look up Carol Burnett show outtakes on Youtube. Conway's Mr Tudball needs a secretary is also hilarious. It is not quite the same version played on TV LOL.
Yes the first run with that skit was stale. Burnett gave Conway the OK to ab lib/improvise for the second recording and like you said caught Korman totally off guard which made the skit one of the all time best TV moments.
That dentist chair skit was also the first time Korman did a skit with Conway. I’ve been taping the Carol Burnett show reruns on MeTV lately and you can tell when Conway goes off script by seeing Korman break character and start chuckling!