Hey Matt,
I'll need a bit more information before I can give you any useful advice. Let me recap what I think you want to do:
You want to create a plugin that will be an optional "up sell" purchase for new customers of your software, but you want to "grandfather in" old customers of you software. The plugin will be a separate download and/or update to your app and its availablility will be controlled by a custom field like "is_pluginX_enabled".
Is that about right?
If it is, then the way to do this is simply create a new custom field ("is_pluginX_enabled", or whatever you want). Then edit all the product keys that you sent to this reseller and set the custom license field to whatever you want (like "true" or "1").
You don't even need to bother the reseller.
Does that help, or have I misunderstood what you're trying to do?