Subscription plan - trial member with upgrade

Hi,
I am offering a guest-member level for a limited time period for a minimum price. The guest member receives the same privileges as a regular member. During or after that period the member should either upgrade to regular member or should fall-back to non-member privileges.

How do I define the subscription plans for this?

Currently I have the membership levels:
.: members-only (with full access)
.: free content

And the subscription plan for guest member:
.: "Members-only", "Finite", "3-months","3 Euro"
.: "free content", "Serial", "1-month","0 Euro"

Question:
.: Is this correct?
.: How can I allow the guest-member to upgrade any time during the 3-month-trial to another subscription plan?

Thanks,
Frank