Managing Multiple Pricing Options for a Membership level

I have a premium membership plan on my website using Paid Memberships Pro that costs $149 (initial payment, not recurring) and expires in 1 year. I want to introduce another pricing option for the same premium membership plan, which costs $99 (initial payment, not recurring) and expires in 6 months.

Here’s what I tried so far:

1. Creating a Second Membership Plan: I created a new membership plan called “6 Months Premium” and priced it at $99. However, since it’s considered a different membership plan, if someone already has the “1 Year Premium” with days left, purchasing the “6 Months Premium” replaces the “1 Year Premium” plan, removing the remaining days instead of adding to them. I understand this happens because they are treated as different membership plans.
2. Using the Payment Plans Addon: I created a payment plan for the “1 Year Premium” membership level called “6 Months Premium” that costs $99 and expires in 6 months. However, the same issue occurs, as it still replaces the existing membership days instead of adding to them.
3. Applying a Discount Code: I created a discount code that reduces the price to $99 and set it to expire after 6 months for the “1 Year Premium”. Yet, this method also results in the same issue, replacing the membership duration with 6 months instead of adding to the existing days.

Despite trying these methods, the problem persists: the membership duration is replaced rather than extended. I need a solution that allows the membership days to be added instead of replaced when a member with an existing plan purchases an additional plan. How can I achieve this?