Here's a fan favourite of mine from another site. Hope you like it!

interesting fact: I made this on 9 Aug 2005, a few weeks before the episode "A Fine Mess" was aired, in which Odd & Yumi did switch places. Interesting, yes?

Body Switcheroo!

"Uh..Jeremie? We've got a problem down here" Ulrich called up from the scanner room. "I'll be right there" he said.
"OK, what's happened?" he asked, walking in from the elevator.
"Jeremie!" Odd cried, rushing up to him. He opened his arms and hugged Jeremy hard, kissing him on the cheek. "Oh Jeremie! Finally I get to touch you!"
"Odd! What's wrong with you?"
"I'm not Odd. It's me, Aelita."
"What!" Jeremie gasped.
"It's true Jeremie. Something happened as Ulrich, Odd and I dematerialised." Ulrich said. "I'm actually Yumi"
"And whose Yumi?" Jeremie asked, Odd/Aelita still hugging him.
"It's me Jer, Odd." Yumi/Odd said.
"So that means Ulrich's stuck in Aelita's body in Lyoko." Jeremie said, pulling out of Aelita/Odd's arms. "This is going to take sometime to fix, and we have to go back to school."
"Oh boy" Yumi/Ulrich said.

"What a day!" Ulrich/Yumi said, stretching. He/she and Jeremie and Odd/Aelita were in the cafeteria eating lunch. "Sissi won't stop hounding me,everyone's talking about you and Odd, and who knows what Odd's been up to."
"Well at least the day's almost over. Then I can put you guys back in your right bodies", Jeremie said, trying to ignore Aelita/Odd's hand on his knee.
"It could have been worse", he said, "You could have had gym today. Then you would have had to go in the boys locker room."
"Yeah, you're right-" Yumi stopped, a look of shock on her face.
"What?" Jeremie asked.
"Today was my gym day."
"You don't think Odd would've..."
At that moment Yumi/Odd came in, whistling cheerfully. He/she grabbed some food and sat down. He/she took a bite of his/her lunch and looked up to see Yumi/Ulrich and Jeremie glaring at him.
"What?" he asked.

"Look, I said I'm sorry" Odd/Yumi said. Yumi/Ulrich glared at him as the gang headed for the Factory. They entered the elevator and went down to the computer room. Jeremie climbed into his chair and said "OK, I think I've found the problem. All you guys have to do now is just climb into the scanners, and leave the rest to me."
"Oh well, it was fun while it lasted" Odd/Yumi said wistfully.
"Odd!" Yumi/Ulrich growled, threatening him with a fist.
"Goodbye Jeremie" Aelita/Odd said sadly, kissing him on the cheek.
All three of them went down to the scan room, climbed in and waited for Jeremy to begin. "Ready? here we go" he said, typing in a command.
The machines began to humm, and then a glow enveloped them. The glow intensified then dissappeared, leaving the machines smoking. With a whoosh they opened. Yumi, Odd and Ulrich fell out, moaning slightly.
Ulrich got up first. "Oh no, it didn't work!" Yumi/Ulrich said. She walked to her body.
"Odd, Odd, you still in there?"
"No, it's still me Ulrich" Ulrich/Yumi said, getting up.
"Ahh! I'm Odd!" Odd's body went. "Let me guess. It's you Jeremie, right?"
Jeremie/ Odd nodded. Odd/Jeremie came walking in and said: "Hey guys, check it out! I'm a genius."

I see" Jeremie/Odd said, typing at the computer. "Now I know what went wrong"
He turned to the others and smiled. "Don't worry guys. I just need to redo the program. By tomorrow we'll be back to normal."
"Tomorrow!" Yumi and Ulrich said, eyes wide in shock.
"Yeah. It's going to take me all night to fix this"
"Umm Jeremie, this isn't going to work out" Yumi/Ulrich said, blushing.
"Huh? Why?" Jeremie asked.
Yumi/Ulrich stammered on, glancing at Ulrich/Yumi who was blushing just as hard.
"Uh.. Remember Jeremie, Yumi doesn't stay at school like us. She, I mean me, would have to go home."
"Sounds good to me." Odd/Jeremyie said, smirking a knowing smile.
"There's also...one other thing" Yumi/Ulrich said. "It has to do with boys and girls being different. Body wise." she said, her face glowing like Ulrich's.
"Didn't bother me during gym" Odd said. Yumi glared at while Ulrich gave him a questioning look.
"Look, you two think it's easy being in Odd's body? It itches and smells"
"Hey!"
"Point is, it's just going to be for one night. If I can take it, hopefully you can.
Anyway, Odd's got a point. Surely you've had to go the bathroom today?"
Yumi shook her head. "I, I was too embarrassed."
"Unfortunately, until tomorrow, there's nothing I can do. You'll just have to stick it out". Yumi and Ulrich glanced at each other, a worried look in their eye.

Next morning, the factory. Yumi/Ulrich and Ulrich/Yumi got there first.
"Uhm hi" Yumi said, blushing and avoiding his eyes. "Hi" Ulrich said, his face the same. The silence became unbearable, and Yumi decided to break it.
"Man, know I now why you don't like sleeping with Odd. Even in Jeremy's body he snores like a tank. I had to gag him to get some sleep."
"Did anyone see them switch rooms?"
"No". Yumi went silent again, then said, "Uh, Ulrich? I had to go shower yesterday"
Ulrich/Yumi blushed. "Uh yeah, me too. It wasn't so bad, was it?"
"No. And going to the bathroom wasn't too hard. But then I had to go do your washing."
'Uh-oh' Ulrich thought
"But that wasn't the worst part." She blushed red. "I, I had a dream about you."
It was that kind of dream. Weird thing though, I dreamt it from your point of view."

Ulrich just stared at her.
He recovered from his shock and said: "Uh, that's ok Yumi, I think I can top that. This morning, during the shower, your body hit it's period." Yumi's face turned red.

"And then your mom explained everything. Everything"

"Oh God!" Yumi groaned, burying her face in her hands.
"Oh well, it least it was only for a day" she said. Her cellphone rang.
"Hello? Yeah Odd, what's up? What! Oh no! I understand. Thanks Odd."
"What?" Ulrich/Yumi asked. "Jeremie's got a cold, and the nurse is keeping him in the infirmary till tomorrow" Yumi/Ulrich said.