How to allow users to update submission and add remaining data?

I am a newborn photographer. I have my clients fill out a contract upon booking, however, a date is not set in stone b/c we do not know when the baby will be born.

Is there anyway to either make a new form with the date and then have it automatically transferred to the contract that they have already signed? Is that possible?

Or should I link two forms together?

Unfortunately it is not possible to update the submission of one form with the data of another form.

You do not have to create a new form to be able to update the date or other fields of another form. You can take advantage of our edit link feature to allow the users to edit their own submission data at a later date.

You need to add {edit-link} tag in form's autoresponder email alert of your form. When the user submit the form, they will receive the email which will have a clickable link. Click on this link will allow the users to edit the submission data again and complete the remaining form fields. The following guide should help you:  http://www.jotform.com/help/40-How-to-Let-Users-Update-Their-Form-Submissions-at-a-Later-Date 

Hope this helps.

Do get back to us if you have any questions.