This info was originally published in the HP logic analyzer memory upgrade thread, but to make it easier for others to find, it deserves a thread of its own (now that it has been confirmed).
Anyway, it's a simple swap of two resistors:
On 16533A: R810=1004 R809=0 (top side, near the Actel chip)
On 16534A: R810=0 R809=1004