This is a quirk of Acrobat 8: It did not take into account the different page scaling as far as the form fields were concerned. If you would have added your text fields before you've added the header/footer using the page scaling option, the form fields would have stayed in the same position they were before - they would not have moved with the scaled content. Acrobat XI does adjust the fields when you add headers/footers with the scaling option.
Did you also select to shrink the document when you updated your headers? You may have scaled the document twice in that case. I would try to see what happens when you don't shrink the document when you update your headers.
Karl Heinz Kremer
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