As I understand it, your preferred partner at time of booking stays with the reservation, and cannot be modified by you (the HHonors member). Hotel staff can "over-ride" that if you wish to change it, or if you have the time, you could probably get Hilton to do so by phone by the night of checkout.

For a while recently, I had stays where the partner field came through blank (probably because I wasn't signed in when starting the reservation process) - one DT clerk who seemed quite up on those things told me that he understood that the controlling factor is one's preferred partner at time of posting.