Here's snippets from my previous endeavour. My template mxd contained a templated DDP feature class/layer. The new DDP contained the same schema. Look up the ESRI docs for any of the methods used below.
import arcpy
import arcpy.mapping as m
mxd = m.MapDocument("mxd_path")
## make sure your saved mxd already had ddp enabled
ddp = mxd.dataDrivenPages
## get the ddp layer
ddp_lyr = m.ListLayers(mxd, "ddp_lyr_name", "df_name")[0]
## replace the datasource with another feature class
ddp_lyr.replaceDataSource("path_to_workspace", "FILEGDB_WORKSPACE", "new_ddp_fc_name")
ddp.refresh()
## to save the mxd with a the updated ddp layer
## or use mxd.saveACopy() to keep your template and save another mxd.
mxd.save()