Well, in at least one case (Spike at your Service) this is actually true.I don't have a source off-hand, but I think it was Meghan or one of the writers who said the ep was originally written with Rarity, but they had to change it to AJ late in the process. Something about her coming off as too mean to spike, though I assume that's meant in the context of the Rarity/Spike interactions that have already been developed.(And yes, I'm on team Rarity-is-Best-Pony, as well... so looking forward to this winter).