To replace a form, we recommend copying the form on the My Forms page (using the copy link on the form itself) and then directing visitors to the new copy of the form. This will leave the original form and results intact and separate. If you like, you can also switch the Directory settings (on the "Share" page) for the two forms around so that the new one takes the place of the old form. This way, visitors can use the same URL/links but see the new form instead. You can even "close" the old form on the "Form Settings->General->Open/Close" page to ensure that it does not accept any more submissions.