If anypony would’ve been close to Lyra’s house when the two drank the potion, they probably would’ve called the guard, since there was a bright blue-purple-ish light beaming out of every window, crack in the walls and the chimney. Lucky for Lyra and Twilight though, nopony was near.
When Lyra opened her eyes again, the first thing she saw was Twilight’s face, distorted by the cup it was reflected from. Once that her mind was fully working, she pulled her face off of the table and stared at the pony on the other side. There she was, looking at her own body, which now belonged to Twilight. Shortly after Twilight rose as well and stared right back. She was the first one to speak.
“This is… interesting. I mean, it felt weird getting wings. Losing them again is even worse.” A shudder went through her body and she shook her head a few times. “So, do you like being an Alicorn?”
“Do I like it? Twilight, this is probably the best feeling I’ve ever had! The Wings, the magical power! This is awe-” She suddenly stopped and made a few moves with her muzzle. “New teeth. That’s weird. Now where was I? Ah, right; This is awesome! Thank you so much for choosing me as your… replacement? Representative? What exactly am I?”
“Well you’re not a replacement, that would be permanent. Representative is also wrong…” Suddenly, a smirk crept onto Twilight’s face. “I think you’re an impersonator. Sooooo, you don’t want anybody to find out about this. The last one who impersonated a princess got blasted out of Canterlot.” Then the smirk vanished again and Twilight shook her head. “I think you’re rubbing off on me. So. During my time away, you’ll have to take over all of my duties. Not only as a princess but also as a friend.”
“You can count on me Twilight.”
“If you need help, Spike is aware of the situation. He’s the only one though. And I’d like it if it would still be like that when I come back. Please, nopony, absolutely nopony is allowed to find out about this. Not my friends, not the princesses, nopony at all!”
“Fine with me. But why is that so important to you?”
“Imagine what would happen if anypony found out! Twilight Sparkle, the Alicorn that couldn’t take it! Celestia would hate me, my friends would abandon me and the masses would laugh at me! You can’t allow that to happen. Under NO circumstances!”
Lyra gave her a deadpanned look and raised a hoof. “I don’t think-”
“Alright. I’ll be on my way now. If you need to contact me, Spike can send letters to me. I’ll see you in a week!” And with a magical ‘poof’ she was gone.
For a moment Lyra just stood there. Then she let out a scream of happiness that echoed over half of Ponyville. Carrot Top, who just so happened to walk by, dropped all of her carrots and jumped a little bit at the sound. Concerned about the fact that there were screams coming out of Lyra’s house, she knocked and entered.
“Lyra? Are you alr- Oh, princess Twilight! I didn’t expect you here. Where’s Lyra? ”
“Lyra just left… for holidays! she’s taking off a few days and asked me to maybe watch over her house a little bit.”
“Oh. Well you as a princess must be very busy and while it was nice of you to help Lyra, I could watch the house.”
“You would do that? Oh thank you so much. I do have a lot of work up my sleeve at the moment. I’ll take my leave now. Thanks again Carrot Top!”
Inside Lyra’s mind:
Alright, time to test this magical power! Teleportation should be a good start. Concentrate… Concentrat-WHOOOAAA
In the real world:
Just as Carrot Top was about to ask what the sound was, Lyra’s body was engulfed in a very bright light and ‘puff’ she was gone. This left a, once again, startled Carrot Top in Lyra’s Dining Room.
Meanwhile, in Twilight’s new castle, an equally bright light appeared and Lyra materialized there. For a moment she just stood there. Then she fell over. “That was awesome! I wanna do that again!”
“That’ll have to wait until later.” Lyra instantly got up and turned around. That had not been Spike, nor any of Twilight’s friends.
“Who is there? Show yourself!” On command a figure emerged literally out of the shadows, slowly taking form, until a figure around twice as tall as herself stood in front of her. She gasped. “Discord!”
“The one and only.”, the draconequus said, as he sat into a chair made out of honey. “So,” His expression got serious and Lyra realised she had done something very, very wrong. “Explain yourself.”