Hi Melissa,
Any existing forms created before we introduced this key field mapping will continue to function as normal, however if you open to edit a form created before this time, you will be unable to re-save that form until you set key field mapping.
So while if you go back and edit a form, you will be required to set the key field mapping, any forms created prior to the key field mapping being included will continue to work without any modification.
Hope this cleared up any confusion. Have a great day!