To change the data source name, you need to check its current value and then issue single rrdtool command.
Check the data source (DS) name.
$ rrdtool info graph_memory.rrd | grep ^ds | sed 's/.*\[\(.*\)\].*/\1/g' | uniq
memFree
$ rrdtool dump graph_memory.rrd | grep name
<name>memFree</name>
Change data source name from memFree to memoryFree.
$ rrdtool tune graph_memory.rrd -r memFree:memoryFree