Was Eevee meant to be the mascot?
In Pokemon Conquest, Eevee was the primary critter for the hero of the game. Long story short, this is not a new move on Nintendo’s part. Eevee is a longtime popular character in the franchise who’s remained topical for most of the series’ history and has been used as a primary Pokemon in side games before.
Did Pokemon change their mascot?
While Pikachu may nowadays be the worldwide mascot of the Pokemon series, the original mascot was actually going to be Clefairy. Game Freak changed the series mascot to Pikachu because they apparently felt that while Clefairy would only appeal to girls, Pikachu would appeal to everyone.