If an Atomic user chooses to switch to Classic after the Calypso editor is deprecated, add a check to see if classic editor enabled on the users Atomic site, if not enable it, and then redirect to the editor in wp-admin
Check out PR to local Calypso dev enviroment.
Open an Atomic sites plugin page and check that the Classic editor is not currently active
In Calypso open the block editor for that Atomic site. Under the floating Help button bottom right click on the ‘Switch to Classic Editor’ button.
Check that you are redirected to the Classic Editor in wp-admin
Go back to Calypso and check that when you choose to edit a page the editor preference has been changed so that you go into the classic editor and not the block editor (at this stage you will still go to the Calypso classic editor, once this is deprecated you will go to classic editor in wp-admin)
N.B. in order to switch back to block editor in Calypso to retest you first need to deactivate the classic editor in wp-admin plugins.