IIRC, the first time the panelist/host swap was done, came during season 1 in 1969. I believe it was Orson Bean knew the little boy who was central character and switch seats with Garry Moore. I could be wrong, but I think it was Orson Bean's idea.
Although a panelist's disqualification from questioning overall, gave the other panelists more time to ask the contestants questions, I have a tape of a 1970/71 episode where Bennett Cerf asked some questions, but then thought he recognized one of the contestants close to where his time was up. So, Mr. Cerf just disqualified himself from voting, thinking it would only be fair. Turns out,the person he thoght was the real one, was actually an imposter! It seemed like he set an example that led to a permanent rule, if I am correct.
Since that time, I'd seen on BUZZR an episode from the original prime-time series where a voting disqualification itself happened with Tom Poston. That would have been years earlier.
I've also wondered, if on TTTT or WML, if there were many occurrences where more than one panelist disqualified him/herself. I did see one occurrence on WML from around 1972.