From the Super Mario Wiki
Transformation Doors are objects found in Yoshi's Woolly World and its Nintendo 3DS port Poochy & Yoshi's Woolly World. They are similar in function to Whirly Gates from Yoshi's New Island; as their name implies, they transform Yoshi while transporting him to another part of the level. Many of these sub-areas often contain stamps, Smiley Flowers and Wonder Wool.
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