I've added and eliminated questions in my Discovery Form. I changed the link for "Discovery" to now point to the new form so that new clients see it. Problem: existing clients now see only the new ...

There is no way around this except if you let the older clients work with the old Discovery form by setting permissions for them to see it, then reactivate it and also link it to a submenu under FORMS (in WEBSITE > MENUS). Also, for the newer Discovery form set permission only for the new clients, so that the old clients will not see it. This way, every one will work with their appropriate Discovery form.