If you are running headers and/or footers, they will maintain their width for the portrait page when you shift to landscape. If this is not what you want, you must first open the header/footer in the section you wish to change, and, if it says, "Same as previous," find the "Link to previous" button in the Header/Footer "extra ribbon" and click on it to remove the link.

For instance, say you are inserting two landscape page in a portrait doc.
Open the hdr/ftr in the landscape section and unlink. Do the same in the following section. Then you can make the change to tabs or margins in the landscape section without affecting the sections before and after.

Best,
Ulodesk