Tesla calls it "a change to the economics of Supercharging," and what it means for current owners is nothing. They will still be able to fill up for free. But, if you order a Tesla after January 1, 2017, your car will be limited to about 1,000 miles (400 kWh) of free Supercharging per year. This is because Tesla wants to encourage Supercharging to be used only for long-distance travel, not for local fill-ups. If you want to Supercharge beyond the 400 kWh in a year, Tesla will charge you a "small fee," which will be detailed later. For now, all that Tesla is saying is that it will "cost less than the price of filling up a comparable gas car." Tesla says that this new policy, "allows us to reinvest in the network, accelerate its growth and bring all owners, current and future, the best Supercharging experience." Unless you think "best" means free, forever, of course.
There are more than 4,600 Superchargers around the world, and anyone who orders a Model S or X still this year and takes delivery before April 1, 2017 will still get the free, forever deal. Apparently, forever ends next April Fool's Day. You can read Tesla's full statement here.