Enjoy215, I am also in the same condition like yours. I got I797B while changing from OPT to H1B. You have two condition, either you can file Change of Status form again and receive I94 or You can go for Stamping. Since your B1 visa is already expired so I dont think you can do it by first way. So you have to go for Stamping. If you talk with any lawyer, let me know as well.