This is a really interesting question. I'd approach it in a different way: You shouldn't focus on a one time contact, rather on creating a funnel for those who signed up to the free version. I'd personally get in touch multiple times, each time using a different messaging according to the stage of the funnel where they're at (and, if you can track the actions they made while using the product, even better!). For ex. I follow up on the 1st day asking what they think, then again 3 days after with a different message. Then I track someone using a specific feature of my product so I get in touch giving tips on how to best use that feature and show them how much they'd be able to do with the paid version instead, etc. Hopefully, this makes sense!🙂