Where: This change applies to Lightning Experience and Salesforce Classic in Professional, Enterprise, Performance, Unlimited, and Database.com editions.
When: This critical update is activated automatically in production orgs on July 11, 2020 and in sandbox orgs when they are created or refreshed.
How: We recommend that you test this update in a sandbox to ensure that the new URL works with firewalls and proxy servers in the network path between your users and Salesforce.
To activate this critical update, from Setup, enter Critical Updates in the Quick Find Box, and select Critical Updates. For Stabilize the Hostname for My Domain URLs in Sandboxes, click Activate.
If you use service provider (SP) initiated SAML to log into your sandbox, work with your identity provider (IdP) to use the sandbox’s stabilized My Domain hostname.
If you have existing sandbox orgs, activate the critical update in those sandbox orgs. Activating this critical update in a production org applies the update to new and refreshed sandbox orgs.