Yes. You will need to close the form to submissions while you are adding the new fields, but your collected data will remain and your new fields will be added to the View Responses tab.
Thank you . . . we’ll try that . . .
Yes, a form can be updated adding new items but there are a few things to be aware of.
1) Be careful editing existing fields or removing existing fields (to delete a field from the form but retain the responses in that column duplicate the column in the response table before you delete the field).
2) If you have also distributed your form as a PDF it is important to know the existing PDFs you have already downloaded may not work any more and that you should download a new PDF after making your changes and re-distribute the new PDF
Adding fields is straight forward and any new responses would (assuming the user entered data for the field) contain the response in that column.
When you go to make an edit you will be reminded that the form will be closed and will inform you of the PDF issue noted above if you have saved a PDF before. Remember to re-open the form after making your changes so the form can receive submissions again.