The low impact version would be to use the existing application to communicate with the users. You can either tell them there's a new version coming (with a lin

The low impact version would be to use the existing application to communicate with the users. You can either tell them there's a new version coming (with a link to a landing page with a call to action) or you can ask them if they want to opt in to announcements regarding the new application. Alternatively you could use a service such as mailchimp to send emails to all 65,000 users. If this is the only time in 4 years that you've emailed them, then I don't think many people would react badly as long as you introduce yourself, let them know why they're getting the email and give them the opportunity to opt out (which they'll get through a service like mailchimp). It also helps if you keep things completely on topic (i.e. "you're receiving this because you signed up to our app and we wanted to tell you about some of the new features we're rolling out in the next version"). If you're getting a 1-2% conversion rate from mail shots you're doing well, so you might want to consider both (I'd probably do the mailshot first, then the page rather than the other way around).