Currently, a subscriber holding one Plus plan (e.g., Mail Plus) cannot purchase or switch to another Plus plan (e.g., VPN Plus) through any self-service path. Checkout fails with 'email already taken' across web, promo pages, and the in-app upgrade button. This forces existing paying customers into either Unlimited (the only working flow) or a manual cancellation-and-refund dance. Please implement: (1) clear messaging when plans conflict, (2) a supported plan-switch or conversion path, (3) checkout that recognizes existing accounts instead of treating them as new signups.