Change Plan link for existing members with the plan and promo code already applied

Let admins send current subscribers a link that opens Change Plan with a chosen price (for example their tier's annual price) and a promo code already applied.

Today a monthly-to-annual campaign depends on each member opening Billing, choosing Change Plan, picking the right price and typing the code themselves, and many miss the code. Sign Up Links can already carry a coupon for new members, but existing subscribers get "You're already a member of these groups!" on them.