It seems this isn't possible from one call, but I'm trying to UNSET the contact key in a REST upsert to customer (removing the Primary Contact) -- I've tried to set null to it, with and without a ?replace=contact, both 200, but the primary contact remains on a GET. I have a feeling I have to delete the Company from the ID of the Primary Contact to get it removed; but wanted to see if I'm just missing something before going down that road?