Unless the programmers made it that way, my guess is that Serene Grace takes the second ability slot and that Togepi uses Serene Grace, even though Hustle is still there. Once migrated...since Serene Grace is in the second slot, the game changes Togepi's primary ability to be Hustle, since Serene Grace now takes the second spot in the ability slots. So in short...to my best knowledge, I don't think Togepi will ever get Serene Grace as an ability.