In theory, yes. There should be nothing stopping you from upgrading the ROM to version 4B (latest) assuming SBIG still have the ROM's available. Considering v4B was released in 1991, your chances are grim. Email
sbig@sbig.com to confirm. I think Linda Sasaki is in charge of the upgrade negotiations.
Failing that... Try a sell the camera and pick up a later version on astromart or get yourself the ST-402ME which will allow you to guide on much fainter stars.