Hi Saroopya,
Congrats for getting admission to Mason. I dont know the procedure of converting H1 to F-1. But there is a group at Orkut
http://www.orkut.co.in/Community.aspx?cmm=28581275 where you can get guidance from the seniors. From the past threads which i'm following, there was a guy who is in the similar situation, he is in US with L1 status and planning to change it to F-1. You can contact him at
http://www.orkut.co.in/Profile.aspx?...04180143282666 .
Otherwise you can contact Reves center, WIilliam and Mary for further information
Hope this may help you
Thanks
Chandu