It doesn't really matter what units the airline uses, miles are just a convenient one. The could measure it in pancakes and it wouldn't make a difference, they'd just adjust the award charts to reflect that.

So, if the airlines are using nautical miles (which I'm tempted to say that they do), and they converted instead to statute miles, you'd get 15% more miles, but the airlines would compensate for this by increasing the award levels 15%.