LL Support tells me that "Linden Home assignments can only be done for Premium Plus accounts that do not currently own a Linden Home. If you are still interested in this Linden Home please abandon your existing Linden Home and reply to this ticket once you have done so."
When I followed this procedure, however, the requested home was assigned to another resident before the LL staff were able to process my request leaving me homeless.
A better procedure would be to allow Premium Plus residents to keep their current home until the new home could be assigned at which point their previous home could be automatically abandoned and items returned.