(New, but equally awesome singer for Luna now!)
If you have ever followed a show for as long as pony has been airing, you know that inevitably, some characters end up losing a voice actor and needing a replacement. We've been lucky so far that such a giant cast of main characters has remained somewhat constant. Off the top of my head, only Luna has a new voice, and it's only in her singing form.
Hypothetical time: Voice actors are busy people, and the one acting for your favorite character isn't available for the next season of pony! Who would you choose to replace their voice? Would you want them to try to mimic the same style as closely as possible, or make up an excuse in the show for the sudden vocal change?
Be sure to include a character that said voice replacement plays so people can more easily relate to them if they don't recognize the persons real name!