How do I change the footer, but not the header?

Hi there.

I'm working with a 475 pages document (in Adobe Acrobat X Pro) which already have a header and a footer (header describes document name, footer is the current page number). I have added some new pages in the middle of the document and want of course to update the page numbers, but how do I do that without changing the document headers?

Kristian, By og Landskab.