I finally found out why I couldn't upgrade my phone online and the info may be helpful to others. Apparently, online sales uses the original zip code you used when you set up your original Verizon contract. For me, that was about 10 years ago. Even though my online login and my most recent Verizon contract uses my current zip code (we moved 6 years ago), I can't upgrade without using the original zip code (even though I can access my account online with my current zip).

So when you log on to verizonwireless.com, change the current zip code in upper left corner to your original zip code and then click upgrade and you'll see your choices. Hope this helps those of you who still can't access online upgrades.