While half the brony community is obsessing over Derpy, we're all missing the obvious reason why her voice changed: time shenanigans!
A quick cartoon I made to explain the changes made to Derpy. Her frequent pairing with Doctor Whooves combined with Doctor Who's new catchphrase "time can be rewritten" gave me the idea for it. Mostly just a fun concept piece, vectors are not that interesting really.
Created with Inkscape.
Creative Commons 3.0 – BY NC ND
Derpy Hooves & Doctor Whooves from My Little Pony: Friendship Is Magic (c) Hasbro