When you install a new visual editor in your Plone Site, you can set wysiwyg_editor in portal_memberdata to the proper value for this new editor. This only means that new users will get this new editor. To change the editor for all existing users, use this package.
This makes available a browser view @@set-default-editor and follow the instructions. You can choose to run this in dry-run mode and see how many users would be changed. When dry run is not selected, details are logged to the instance log.
On the same form you can also select to set the default editor for new users at the same time as changing it for existing users.
When there are many users (see Site Setup / Users and Groups / Settings) we try to get all members by searching for a login with 'a', then 'b', etc. This is not ideal, but if for example LDAP gives problems because it returns too many results (which means it does not actually return anything at all) then this may help. If you still have trouble, try setting event-log-level=debug in the instance section of your buildout.cfg, run buildout again, start the instance in the foreground and then try again and see where it goes wrong. Hopefully this gives you an idea for some custom code to write. (If it is general enough, please share your work in a branch and contact me.)