Upgrading form xml after publishing

[FondaUK]

I have an xml file containing data similar to those below
<Survey>
<QuestionID>1</QuestionID>
<Question>Date Joined?</Question>
<Date></Date>
</Survey>
<Survey>
<QuestionID>2</QuestionID>
<Question>Date Moved?</Question>
<Date></Date>
</Survey>

I have created a form using this xml document as the basis for my form,
published it a sharepoint folder and have several xml docs saved in this
folder using the form.

I now need to add several new questions to be answered into the form. Fine,
I extract the files, update the files with the new questions, republish - all
ok. New questions showing in new forms created.

How do i get these new questions to appear in the files that have already
been saved on the sharepoint site using the original form?

Making changes to the form, such as colours and layouts will appear in the
files saved using the orginal form, but not the new questions.....

What am I doing wrong?

Many Thanks