Subscription Update Preview API

I want an API endpoint or an enhancement to the existing subscription update API that allows me to preview the financial impact of a change (proration amounts, new recurring totals, credits) before actually performing the update. This is important so I can show customers exactly what they will be charged or credited (e.g., "You'll be charged €808.05 now. Your new recurring total will be €842.52/year") when they switch tiers or billing cycles.