Why didn't they just have Annie switch sides in the third season to make up for Lisa's departure?

I still hold the opinion that the third season is when things really fell off of a cliff with Lisa being absent, the weird change in the formula (by having those animated characters serve as "hosts"/play-by-play commentators), and the needless additions of Marcus (who really screwed up the dynamic) and Gilda. I mean they already had an episode in which Annie and Lisa switched personalities (a la Freaky Friday) so it isn't that much of a stretch to imagine seeing a Prankster "work" with the Electric Company.