The ballparks have basically been the same since the beginning. A couple of years ago I believe they tweaked some of the ballpark ratings to adjust them closer to updated ballpark effects, but I don't believe they added any new parks at that time.
Most of the parks are real major league or minor league ballparks, but there are some exceptions. There are a handful of ballparks that are purely fictional (Madison, Cheyenne, and Santa Fe; there may be others). And Hartford's Welch Park is actually a Little League / Softball complex in a suburb of Hartford.