This question already has an answer here:
I would like to enter the USA on the Visa Waiver programme, and after a week of stay change over to a J-1 visa (i.e., have a week of holidays before I begin my academic exchange).
Is this possible without leaving and re-entering the U.S.?
Edit: it seems in other threads that some people have left and re-entered, that there seems to be a 30-grace period for early entry of J-1 holders, and that conversion from one visa to another is a lengthy process. Has anyone made any experience with these options?