(SPOILER) For those who are really confused who switched with whom.
1-6) Major Mare
2-16) Mr. Cake
3-12) Ms. Cake
4-18) Big Macintosh
5-10) Granny Smith
6-5) Bulk Biceps
7-17) Lyra
8-3) Bon Bon
9-1) Derpy
10-15) DJ-Pon3
11-4) Octavia
12-14) Berry Punch
13-10) Dr. Whooves
14-13) Rose
15-8) Mr. Waddle
16-2) Colgate
17-7) White Lighting
18-9) Daisy

Author's Note

Bear in mind that these were the only background ponies I mentioned in the story, and it doesn't mean that the others were not affected. :L