We did something similar. After our rebranding, we decided to release a new app on Google Play and send out emails to all our old users asking them to migrate from the old app to the new app. Obviously, as expected, a lot of people didn’t do it because the friction was too high. In our case, previously they had the app as well as the web version, but with the new app, it was app only. So a lot of people complained about that. Another unforseen impact was that many people realized that they didn’t use the app, so they uninstalled it and then just because they got an email, gave us 1 star rating on the old app(thankfully not on the new one). Another experiement we did was A-B testing with the email content. Emails that had members first names used more than once had a better conversion
First set of emails might not get you a good conversion, but once you get the initial set of people in, in your next set of emails you could include topics that they are talking about in the community with a CTA click here to join conversation. We found these to be super effective.