The Hidden Math of Mid-Cycle Subscription Upgrades

Short summary

Mid-cycle subscription plan changes require charging the difference between the new plan's prorated cost and the customer's unused credit—not the new full price. Handle four distinct seat-change scenarios separately (upgrade+seats, upgrade fewer seats, downgrade, first purchase), normalize billing dates to prevent timestamp drift, and log all inputs for observability and chargeback disputes.

• •Mid-cycle charge = (new plan prorated cost) − (unused credit on old plan), not new full price
• •Four distinct cases by seat change: upgrade+seats, upgrade−seats, downgrade, first purchase—each requires different logic
• •Use dates not timestamps for billing boundaries to prevent double-bills and gaps; aggressive logging enables chargeback defense

Generated with AI, which can make mistakes.