Thanks for your help...but the headers aren't variables...
It may be that your solution is to bite the bullet today and do the page-by-page change of your headers to variables.
System header variables can, for example, contain text from a paragraph style such as a level-[ 1 | 2 | 3...] heading. Or other values received from a FrameMaker named paragraph style.
User-defined variables can also be used in headers and footers, although they can't be made to do the zippy automatic things system header/footer variables do...
The essential point being that after doing it the hard way today, you may never need to do it again tomorrow. Or, alternatively, subsequent changes can be effected by changing the value of the variable and importing that into the desired target chapter files...
Cheers & hope this helps,
Riley