Has this already been done? The Comedy Blow to the Head, where somebody is knocked out in a fight or by accident, and wakes up with a wholly new personality often radically at odds to the person they were before. Often only another Comedy Blow on the Head in the same place restores them as good as old...
A couple to start you off.
- Dragon Ball has this in the protagonist's backstory. When he first landed on Earth Goku was a typical, Always Chaotic Evil Saiyian, causing his adopted grandfather no end of trouble. One quick tumble down a cliff and landing on his head later, he's the happy-go-lucky protagonist all minions of evil will come to know and fear.
- Christopher Titus joked about how if anyone saw his kids running around crazy in a store and he wasn't doing anything to keep them in line, they had permission to thwack them on the top of the head cause "It's still soft there and it's like a reset button."
- Sergeant Dori Doreau in Sledgehammer gets one of these. When the formerly liberal and gentle-minded policewoman wakes up, she has become a carbon copy of Sledge. Her impeccable dress sense goes and she starts to affect ill-fitting garish jackets and sunglasses and carrying an unfeasibly large gun. Captain Trunk is delighted.
- In short-lived sitcom Terry And Julian, the very gay Julian Clary takes a Comedy Blow To The Head and wakes up offensively and unmistakeable heterosexual, becoming the worst type of chav leery "lad".
- An episode of My Name Is Earl made a joke about Earl knocking Randy in the back of the head and he became an instant genius, only to revert back to his stupid nature upon a second blow.
- In [[Series: It Ain'tHalfHotMum]], the very effeminate and pacifistic Gunner Beaumont - a female impersonator who would have won applause from Klinger in MASH - gets knocked out in a grenade explosion and wakes up as a fihting maniac demanding to be alllowed out on patrol to kill Japanese . Sergeant Major Williams resolves not to waste this and takes him up jungle...
- Batman: Supervillain King Tut is a perfectly nice professor of Egyptology, except when he gets a tap on the head he thinks he's Tut. Practically every one of his appearances ends with him getting another knock on the noggin which switches him back. Neither personality knows about the other.
- Charles In Charge: In one episode a tap turns Charles into Chaz, a slimy-smooth ladies man with a personality nothing like Charles's. Chaz gets married before he reverts to Charles. Everyone is worried that he won't be able to get an annullment, but it turns out the marriage isn't valid: Chaz didn't fill in his last name on the certificate.
- Happened periodically in The Flintstones. Often Fred would go for most of an episode with the altered persona, his friends increasingly bemused at his change in behaviour, until some object happened to fall on him.