If I understand you correctly, you want to change the plug-ins source location on an old image to use the newer targets?
Plugins just come from OPKG so look in /etc/opkg/ but don't be surprised if you break something should you try to change then install something as dependencies will be changed/different and it's not just the plugins, almost everything comes down through opkg which is what provides the 'online' update, and of course, it would be nigh on impossible to offer any support in that configuration.