Pessimistic assumption: Harry's wand is not already touching a suitable object for Transfiguration. Neither partial Transfiguration nor extremely fast Transfiguration of extremely small volumes lift the restriction against Transfiguring air, dust specks or surface films would need to be specifically seen, the tip of the wand is not touching his skin, and the definition of "touching the wand" starts at the boundary of the wand material.