A few moments ago Brewer PR manager John Steinmiller posted the following on Twitter:
Green grass at Miller Park. Does anyone notice something different on the field?! @ Miller Park instagr.am/p/RK63eBx_gS/— John H. Steinmiller (@jstein1981) October 24, 2012
Some of you are more detail-oriented than I am and will spot the difference right away, of course, but if you can't put your finger on the change perhaps this Michael Cuddyer picture from 2011 will help you.
Kudos to Brett Christopherson of the Appleton Post Crescent for being the first person I saw ask what happened to the dirt walkways from the dugouts to home plate. He also asked Steinmiller if any explanation has been given for the change, but as of this writing I haven't seen a response.
The dirt walkways haven't always been there: The picture in this Baseball Pilgrimages post from sometime in the ballpark's early years doesn't have them. The details in that picture are a little fuzzy but I think it was shot during this game against the Red Sox on June 8, 2003.