But you don't need to wait for Twitter to upgrade your account to take advantage of this. Instead, you'll just have to jump through a couple of hoops.
Twitter user Rob Graham (@ErrataRob) posted the following tweets, which explain exactly how to post 280-character tweets now.
It's tricky and involves a bit of changing code in your browser and using the command line interface on your computer. Even your author got lost along the way, though it appears to work if you're willing to put in the effort and have a bit of know-how.
If you don't want to jump through these hoops, you can wait until Twitter enables your account to send 280-character tweets.