I'm in the same boat, except I have an '01 ES, and my Chilton's warns me like an angry mother every time you do a procedure in which the battery must be disconnected, that you should make sure to have the code.
I've gone through my owner's manual, and it doesn't say anything about radio anti-theft codes. I downloaded the Lexus factory manuals from techinfo.lexus.com, and so far I'm on page 350 or so out of ~2000, and it doesn't seem to say anything about changing codes. Just that you should have the code when you disconnect the battery. <_<
Any help? Maybe if you post the procedure for your '92, it will work on mine.