Opt Into Beta
Here’s a technique I’ve used on launches: instead of a big scary waterfall, or, in tandem with a phased release, let customers opt into a new feature.
On the first step of your legacy app, add a link to the new app that says “Try our new beta app!” Then, add add a link to the new app that says “Go back to the legacy app”, and track clicks to it.
The benefit is that some early adopters will try the new app voluntarily, and their tolerance for issues will be high because they’ll think “I chose to opt into this and it’s a beta feature.” You might even get some free, friendly bug reports.