How can I change a form (have a newer version of it) and still keep the answers the clients gave to the previous version?

When you need another version of a form to which clients already gave answers, a better practice is to create a new form and deactivate the previous one, if you no longer need it, but want to keep the existing answers. You can make copy the previous form and edit it as needed. Setup permissions and assign it to a new sub/menu. Your clients will be able to work with this new form and you will still have their answers to the previous version of the form on your coaching end in CONTACTS TAB > CONTACT NAME > "Forms" view all.