The Stats module of the Jetpack Plugin does not appear to import data from the WordPress.com Stats Plugin that it replaces.
From the JetPack website FAQ:
As we upgrade each of our individual plugins to be a part of Jetpack, we’ll prompt you to switch over to the new, Jetpack-powered version. Don’t worry about upgrading, we’ll automatically take care of carefully switching over all of your settings and options so that it’s the smoothest experience possible. Once you’ve upgraded to Jetpack, you can remove the older versions of those plugins.
When I activated Jetpack, it (apparently) automatically deactivated the WordPress.com Stats Plugin. Secure in my assumption that this FAQ entry would hold true, then deleted the stand-alone version of WordPress.com Stats.
I now have no historical stats data.
My site is mostly a hobbyist site, so this loss is mostly an annoyance. Others might find it to be a considerably bigger problem.