Access On Premise Apps Using Entra Identity

Yes, you can still access your on-premise applications using Microsoft Entra ID, even after switching to Microsoft 365 with a new domain. Here are a few ways to make it work:

• Entra Application Proxy
  This lets you publish on-prem apps to the internet securely. Users sign in with Entra ID, and you can set up single sign-on (SSO) if needed.
• Hybrid Identity with Entra Connect
  If you sync your on-prem AD to Entra ID (using tools like Entra Connect), users can keep using their old credentials, and apps continue to work. Even if your primary email domain changes, you can still keep the old domain in the background for compatibility.
• Keep the Old Domain in AD
  As long as the old domain still exists in your local AD and is synced or trusted, users can still access apps tied to it. You can add both domains to Entra ID if needed.

In short, as long as the identity behind the apps is still valid (even if the domain changed), and you set up syncing or a proxy, your access should work.