If a Fairy type does exist and past-gen Pokemon gain or are retconned into the type, I would expect only the Normal type to be replaced by Fairy in those instances. I don't think dual-type Pokemon without Normal as one of their types will drop one for Fairy.
Clefairy and Snubbull are the obvious ones... Uxie, Mesprit and Azelf would probably change to Fairy. Not sure about Gens 3 and 5. (Meloetta is a muse, not a fairy.) Celebi and Manaphy would probably get Fairy too.
Where are people getting Glalie from?
glalie (and more so snorunt) could just be the fact that it looks like a bit of spirit which i guess could be a fairy. kinda my thought process anyway. and i like that idea for snorunt and glalie.
Well that and they are in the Fairy egg group. I think Glalie would make a pretty cool malevolent fairy.
http://i.imgur.com/ua00T9H.gif http://i.imgur.com/gw03gl7.png - Number of people that D'awwed: 75