I wouldn't be surprised if mono-typed Pokemon are the only ones retconned to fairy either. The only dual type ones I could see getting retconned to fairy are the Togepi line, which might have Fairy replace Normal.
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Could also easily see Celebi being Fairy/Grass and Jirachi Fairy/Steel.
Nah, that would totally change these Pokemon. At least adding Fairy doesn't remove STAB or (for the most part) remove strengths/weaknesses. Removing a major type like Psychic would basically eliminate an entire Pokemon forever and replace it with a brand new one that happens to have the same sprite and name. Getting rid of Normal is the only type that makes sense in terms of dual-types.
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