Our upgrade window was yesterday (Thursday). Like jbpats saw the day before, I was surprised at the number of sections with tickets (almost every section), though in many cases they were single seats, or three seats (and the Celtics don't allow you to leave a single empty seat), so a lot of sections showed green, but didn't have anything.
I did end up upgrading, but I have to say it was a frustrating process. The website took forever, and there were multiple times that I selected seats, but when I attempted to complete the order, it told me that the seats were no longer available.
At the end of the day, I selected seats in a section I was unfamiliar with, and didn't have time to check the sightlines (fear of losing yet another set of tickets), so I'm hoping I don't regret it...
Final note on the upgrade process - I had a friend who was on the waitlist, and while he was offered seats (his selection window was the 2nd week of March), they were endzone balcony, so he didn't pull the trigger. Yesterday, the Celtics came back around and offered him the chance to purchase the seats that the Season Ticketholders didn't take/turned in. Seems like turning down the initial offer was the right thing to do...