According to the article, the upgrade will get you "a higher resolution and improved framerates."
For me, that upgrade is not worth it. I will play the game entirely in portable mode and my vision is not what it used to be, so I don't care about resolution.
So it may be worth $10 for some people, then. I am fine with a cheaper game with slightly worse graphics. I am not sure I even notice if a game is running at 60 fps or 30 fps. Even when a game dips below 30, I notice it getting a bit choppy, but a game should be engaging enough that I will not pay it any attention.
They know that someone is about to spoil the release date, so they quickly throw together a tweet to control the message and buy time for a more comprehensive and thought out announcement as part of the Nintendo Direct.