>>> import arcpy
... mxd = arcpy.mapping.MapDocument(r"P:\filepath_of_my_map_document.mxd")
... mxd.findAndReplaceWorkspacePaths("C:\Users\MyUsername\AppData\Roaming\ESRI\Desktop10.3\ArcCatalog\old server.sde", r"C:\Users\MyUsername\AppData\Roaming\ESRI\Desktop10.3\ArcCatalog\new server.sde")
... mxd.saveACopy(r"P:\filepath_of_my_map_document.mxd2.mxd")
... del mxd