Channard Cenobite: And to think... I hesitated.
Kirsty: You have got to destroy that mattress!
Pinhead: Your suffering will be legendary, even in hell.
Channard Cenobite: I recommend... amputation.
Kyle MacRae: I'll get your clothes. I can do that. I'm a doctor.
Pinhead: It is not hands that summon us. It is desire.
Julia: Take your best shot, Snow White!
Answer: In general, the Channard cenobite seems different from the others. Ex. He seems directly linked to Leviathan through the (rather phallic) stalk/tube. It's entirely possible that his transformation was different all around, and keeping his memory was part of his individual process. Also, there's nothing that necessarily indicates the other cenobites immediately forgot their past lives. It's entirely possible they forgot them over time as they became more acquainted with hell and their new "lives" as "explorers" in the realm of pleasure and pain.
TedStixon