Steps
1. Add button to the WikiEditor and VisualEditor toolbars for switching back and forth mid-edit – T49779, blocked by T104663
2. Change the VE-MW integration to not add a new tab, make it look like the "Edit" tab is active; on page load, re-write the URL of the "edit" button to go to VE or WE based on users' last choice (site default applies if no pref, or for anons, no cookie) – T114530
3. Intercept all calls to action=edit (undo, diffs, //etc.//) to apply consistently for user expectation – T114531
4. For wikis where VE isn't the default choice, on an IP's edit-click, set a cookie of a random number, and allow direction to WE or VE based on some threshold (so ramped-up change is possible) – X
5. For such wikis, slowly increase the switchover with testing to ensure it's not disruptive – X.