Arcpy's replaceDataSource Error

I know this is pretty old, but I just encountered this same issue and was surprised by the lack of solutions out there. The solution is so simple, but you'd have no way of knowing it based on ESRI's practically non-existent help:

Don't include the shapefile extension. ...That's it.

newSource = os.path.join(currDir, lyrZones + "_Dep" + dirNum + ".shp")

Should read:

newSource = os.path.join(currDir, '{}_Dep{}'.format(lyrZones, dirNum))