I have recently begun to migrate my old Gentoo i386 to a Centos-7 x86_64. (This explains the change design blog)
You must export the munin rrd to a xml format :
cd /var/lib/munin/blog.yacoubi.fr/ for i in `ls *.rrd`; do rrdtool dump $i > $i.xml; done
To the new server, you just have to import and convert xml file to rrd file :
cd /var/lib/munin/blog.yacoubi.fr/ for i in `ls *.xml`; do rrdtool restore $i `echo $i |sed s/.xml//g`; done rm -v *.xml
Of course these commands are valid for all rrd file. (cacti for example)