Nope!
It means you will be asked to wait until next month. Next month the Mint team will release upgrade institutions from Mint 17.3 to Mint 18
The instructions last time amounted to"say Please". it was about that easy.
But if your some kind of OS nut,
(if only at very superficial levels...)
Like me. You can download and install Mint 18 this very minute. This choice allows....ok it makes you, install Mint 18 from scratch.
But hey, you'd want to do that anyway right? As a sigh of respect or perhaps a sigh of an unbalanced mind!
Both are fine things to have! ":O}
I leave as the Mint team has left this thrilling choice to your every hand.":O}