Upgrade Free Members in Authorize.net

I have a large free subscriber user base and want to allow them to become paid subscribers along with allowing brand new users to subscribe and sign up. I have no problem having the payment process go through if the user is brand new, however the program doesn’t recognize when free subscribers sign up for the paid subscription.

I know there is a modification button that is easily generated that I can use. But, how can I make it so the pro forms sign up link will automatically know when clicked to direct a person to the upgrade membership pro form or the brand new user paid form?

I’d like to have it that way to make it easiest for all users.

Thanks for your help!