Receiver Jordy Nelson started the day as a free agent. He continues to be a free agent.
His ceremonial retirement with the Packers did not include signing a meaningless one-day contract because, frankly, signing a one-day contract does have some meaning — more meaning actually than one share of Packers stock. (Cue the Packers shareholders who resent having the meaninglessness of their framed piece of memorabilia pointed out.)
By signing an actual one-day contract and retiring, a player lands on the reserve/retired list. The team holds his rights, and can control his rights.