What is the relationship between a Payment Plan and a Fee Schedule?
Payment Plans are created at the member level when you assign a Membership; Fee Schedules belong to Memberships.
When you create a Payment Plan it uses the Membership's Fee Schedule to determine the correct subscription price/fee (based on Billing Date). This is true for initial Payment Plans as well as Renewals.
Because the Fee Schedule allows you to hold multiple fees with different dates, you can reflect [annual] Membership fee increases which will be picked up when you renew the Payment Plan.
But, if you change a fee schedule it will NOT change existing Payment Plans.
So, if you want to reflect an annual membership increase, add a new Fee Schedule line to the Membership (your membership year start-date as the Effective Date) before you run your membership renewals.