For the first four seasons, Jerry had an electric stove. Beginning this season, he has a gas range. In this episode, Kramer lights a cigar off the stove, so the switch might have been made simply to accommodate that scene.
It is also possible that the stove was changed for reasons of product placement. In Season 5, with the popularity of the show, we begin to see familiar products prominently displayed. Kramer lighting the cigar may simply have been to draw attention to the fact that Jerry's stove is gas.