Charzard Picks up a Super Mushroom

[Galvanic]

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...But what if the Trainer decides to switch?

Will Squirtle and Ivysaur become big, too?
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[Fossilized Ursidae]

Most likely not as they didn't get it themselves. Zelda/Sheik kept item transformations, but they are the same person so it's a different situation. We don't know for sure, but I'm guessing that when you switch pokemon you will lose any item you're carrying and/or and item effect.

 

[Percon]

Good question. I agree with the above poster, though.

I guess we'l have to see when the game comes out.