seems i recall a post about this from some time in the past. but overall dolphin is not designed to require a purchased membership from the join form. i think you could force a redirect upon successful join to the member upgrade page, and provide that method of upgrading after they have joined. but the member needs to be in the database in order to upgrade i do believe.
i am going to search the forum, i am sure you done that already, but i will give it a look.
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