Upgrade Configuration and how to correctly adjust amount due

I have this use case:   A user in an Adult Membership Plan for $25/year, and I want to provide them the ability to upgrade to a $100/year plan.   When they upgrade, I only need to collect the difference $75 since it's the current active term.    What configuration setting do I need to look at to achieve this?

I have 3 normal annual plans:

• Youth $10/year
• Adult $25/year
• Family $50/year
• Free Plan

I have this NEW temporary one-term plan $100/year (does not renew)

Current Adult Plan Setting:

• Regular Validity:  This calendar year
• Start Date:  07-01

Workflow Setting (Subscriptions, upgrades, workflows) - on new Special Plan

• Allow upgrade to this: Yes 
• Propose a plan for upgrades only if it is more expensive - YES
• First static condition - This condition must be met to allow upgrades (I have selected Adult Plan and Free Plan ) - upgrade is only permitted if you have these two plans
• Must be state:  Active
• Or can be purchased same time: No, those plans(s) must already exist
• Conditions on latest user subscriptions: NO
• Conditions on latest completed basket: NO
• Date A, Field A or Value A: is not relevant

So, to clarify:

1. If someone is on a currently active Adult Plan, having paid $20, I want them to be able to upgrade to the Special $100/year plan, and they only need to pay the difference ($80)
2/ If someone is on the free plan, they can upgrade to all plans, including this Special one and obviously pay $100/year.

Thanks!
 

First step to allow for upgrade flow make sure all your plans are marked Exclusive as Non-Exclusive would just result in having 2 plans. Will address your 2 questions directly below.

1. If someone is on a currently active Adult Plan, having paid $20, I want them to be able to upgrade to the Special $100/year plan, and they only need to pay the difference ($80)

Ensure your Special plan has prorating enabled. This is configured under the Pricing tab.

2/ If someone is on the free plan, they can upgrade to all plans, including this Special one and obviously pay $100/year.

Ensure all paid plans have upgrades enabled, but ensure the free plan does not have upgrades enabled. This is configured under the Workflows tab.

This should be sufficient. You shouldn't need conditions.

Thanks, Kyle.

Just to double-check:   You mentioned enabling the prorate option on the Special Plan, the user can upgrade to, but the tooltip says, "Take into account the remaining value of this plan when upgrading to another plan."
The way this reads, it seems like I should be enabling the prorating on the plan the user is upgrading FROM instead of TO or have I misunderstood the tooltip?