Invite Plugin - allow invite to existing user

I need to use the CB Invite plugin to invite users to join a CBSubs plan. This works great if the recipient does not already have a registered account. If they do, however, I get the message "Invite failed to save! Error: To address is already a user."

The invitation is to join a higher level membership, so many of those for the invitation already have accounts.

How can I disable the feature where is checks to see if the To: email is existing?

That's not the purpose of the plugin at all so for that to work you'll probably need to modify it to fit your needs. You can't disable that functionality, because it's not designed to send invites to users already registered since for the purpose of the plugin that makes no sense. You'd have to modify the plugin and remove that check entirely.