This is the way I handled it - maybe someone can streamline this.
#
# Modify definition query to specific layers in map document and
# save the modified document to disk.
#
import arcpy
# map document to modify
mapdoc = r"\\hcgis2\e\Arc\Projects\GIS\basemap - All districts for selected year\basemap.mxd"
oldyr = "2014" # modify this...
newyr = "2015" # ...to this...
# ...within these layers - make all lower case
lyrs = {"airport",
"community college",
"city",
"fire",
"school",
"hospital",
"library",
"mass transit",
"multi twp assess",
"park",
"road district",
"sanitary",
"special",
"tiff"}
# open map document to modify
mxd = arcpy.mapping.MapDocument(mapdoc)
# scan through all map layers
for maplyr in arcpy.mapping.ListLayers(mxd):
ml = maplyr.name.lower()
# if layer to modify...
if ml in lyrs:
# ...then update the year within the definition query
print "Processing: ", ml
maplyr.definitionQuery = maplyr.definitionQuery.replace(oldyr, newyr)
# save changes to map document
mxd.save()
# clean up
print "Finished"
del mxd, oldyr, newyr, lyrs, ml