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 users to the new copy of the form. This will leave the original form and results intact and separate. If you would 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 users can use the same URL/links but would end up on 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.