I just tried to rename a graph ("CPU Load" => "CPU load") that I created for a template but got the following error:

"Graph [ CPU load ]: already exists on [ <hostname> ]"

This apparently happens because Zabbix considers the two names identical because only the case of one letter changed.
A workaround is to first rename the graph to something like "CPU Loadx" and then back to "CPU load" but it would be better
if the Zabbix check would be turned into a case-sensitive one.