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Mega Charizard-Y A+ to A: The reason Zard Y was moved to A+ was because of its offensive wallbreaking set and its defensive set. Since its defensive set's sole purpose was to deal with mega mawile, that set is useless now(well, not useless, but much much less useful) and Zard Y's only role now...
However, using trick with choice items allows you to retain the boost and switch moves. I'm pretty sure that choice items are treated differently compared to something like lefties.