The San Francisco 49ers may turn to their greatest rival to address one of their few needs.Seattle free-agent cornerback Walter Thurmond tweeted Thursday that he is headed to San Francisco for a visit. He visited Jacksonville on Thursday.Thurmond is a versatile player who can play inside and outside. He'd be an upgrade for the 49ers at the cornerback position, which is still somewhat of a work in progress.Interestingly, Thurmond and 49ers free-agent cornerback Tarell Brown have the same agent.