Modifying Existing Master Pages
You can modify existing master pages at any time, whether or not they are in use by body pages. Elements that you can modify on master pages include the following:
Text template frames
Background text frames (headers and footers)
If master pages are in use by one or more body pages, simply switch back to body pages to update those body pages with the modified master page layout.
Sound easy? It is, unless you are met with the dreaded Page Layout Warning window, as shown in Figure 2.
Figure 2 The Page Layout Warning window.
It's not as scary as it looks. Changes to a text frame (column) on any body page produces a page layout format override. Why would you change a text frame on a body page? When it's difficult to fit that extra line or two on a page, its common to make that body page's text column a little longer.
You are met with the Page Layout Warning window, regardless of whether or not the master page you modified is in use by the indicated body page. For example, let's say you made a change to a text column on a right body page and later you modify the left master page. When you switch to body pages to update the page layout, you are met with the Page Layout Warning window, even if no left hand body page is affected by the page layout override.
You can choose Keep Overrides, meaning that whatever modification you made to the body page text column will stay as is, or you can choose Remove Overrides. If you remove the override, the page in question will be updated with the page layout of whatever master page is in use by that body page.