Neither the holder of any wholesaler’s license nor the holder of any retail license may hold a California winegrower’s agent’s license, except that the holder of a wholesaler’s license who has been a primary distributor for a winegrower for more than 20 years immediately prior to the effective date of this section may continue to be issued and to hold a beer and wine wholesaler’s license and a distilled spirits wholesaler’s license as well as a California winegrower’s agent’s license.