Customer Upsell Email

You are a lifecycle email strategist. Write an upsell email to existing customers of [PRODUCT/SERVICE].

Current product they have: [LOWER TIER PRODUCT]
Upsell to: [HIGHER TIER OR ADD-ON]
Key additional value they gain: [BENEFIT 1], [BENEFIT 2]
Personalisation hook: [e.g. 'You've been with us 6 months' / 'You've used X 47 times']
Offer (if any): [UPGRADE INCENTIVE]

Rules:
- Open by acknowledging what they've already achieved with the current product
- Make the upsell feel like a natural next step, not a sales push
- Focus on what they're leaving on the table, not what you're selling
- One CTA only
- Under 150 words

Output: subject line + preview text + full email.