"Upgrading from 32 bit to 64 bit mostly works fine with a couple caveats - your 32 bit RRD data is invalid on the 64 bit version and will have to be deleted by running 'rm -rf /var/db/rrd*'. You will lose all RRD history, this cannot be converted."

However, after an upgrade from 32bit to 64bit, you can do the following to convert your graphs:

You will need two linux machines, one 32bit, one 64bit, both with rrdtool installed.
Copy the .rrd files in /var/db/rrd from the pfsense box to the 32bit linux machine.
On the 32bit linux machine in the directory you copied the files to, run:

for i in `ls *. rrd`; do rrdtool dump $i > $i.xml; done

Transfer the resulting xml files to the 64bit linux machine and run:

for i in `ls *.xml`; do rrdtool restore $i `echo $i |sed s/.xml//g`; done

Now you have a bunch of 64bit .rrd files you can put in the /var/db/rrd directory on the pfsense box. They will be updated automatically!

Courtesy of http://slog.carlheaton.co.uk/index.php/2009/10/converting-32bit-rrd-to-64bit-rrd-moving-cacti-between-architectures/