In this case I bring a python utility to merge to rrd files, a very useful utility if you need conserve the old data with your new RRD files.
First download the script from my web –> merge-rrd.tar
Now extract and merge your rrd files.
./merged-rrd.py <OLD_RRD> <NEW_RRD> <FINAL_RRD>
And this is all, a very simply utility to merge two rrd files.