HELP! How to migrate users to new product?

What process steps would you take to migrate users to a new version of your SaaS?

• Technology considerations
• User Communications
• Pitfalls

###Background

My B2C SaaS integration service (think 2-way Zapier) is being rewritten from the ground up to enable features I was unable to offer 10 years ago when I started.

In order to not just flip a switch and kill the existing service, I'm thinking it better to add a new subdomain (e.g. app2.example.com) and have users re-authenticate their integrated apps via Oauth with new our new API Keys.

While this will require about 5 min of new setup, it will allow us to move people over more gradually and have a somewhat proper beta period (although can't really afford running two infrastructures for long).

I didn't find much guidance for this situation (no, I didn't ask ChatGPT), so I'm seeking advice from those of you understand solo-saas.