With this plugin you only click on image near the graph you want to convert, and all the dirty work is made by script.
Installation is simple, like any other plugin - just copy its folder in plugins directory, enable it in Plugin Management, then give rights to users in User Management, then go to Settings/Misc, check settings to be like in the screenshot and hit Save:
Hit save for one time even if settings from the beginning are totally like in the screenshot - just in case.
Also you can choose graphs to convert in the “Graph Management” page. Just select needed graphs, choose a “Fix 64bit Counters” action in dropdown and hit “Ok”.
If graphs could not be converted, you will receive corresponding message and when you press “Continue” it will fix all the graphs that it can fix.

Fixing process is performed after the next run of poller (this is due to the need to edit rrd file), so 64bit counters are begin to work only after two polls. Checks are rather thorough, plugin checks SNMP version (it should be 2 or greater) and every SNMP query constraints.
May be this plugin would save a day or two for someone.
Icon is from FamFamFam Silk Icons.
ChangeLog
--- 0.5 ---
Cacti 1.0 compatibility
--- 0.4 ---
Updated graphs spiking fix (thanks, mitchese!)
update_poller_cache fix for Cacti 8.8.a (thanks, mitchese!)
--- 0.3 ---
Added mass graph fixing through the “Graph Management” page.
Uninstall and then reinstall plugin to use this functionality.
--- 0.2 ---
Replaced short tags "<?" with long tags "<?php"
Code cleanup
--- 0.1 ---
Initial release
Plugin page: http://docs.cacti.net/userplugin:fix64bit
Plugin github page: https://github.com/simnv/fix64bit