Go to leaf's website and download the latest flash update for the TyTn. Load it on your Imate JasJam, and the Internet Sharing should be available again.
Note that you will lose EVERYTHING on the phone that is not on the MicroSD card, including contacts & sms's, so sync first.
The Tytn is a Herm200, a different exterior hardware configuration to the Herm100 JasJam, but the same inside. Will work, no worries.