Magento – How to change page title of a category programmatically

$category = Mage::getModel('catalog/category')->load($id);

$catnames = array();
foreach ($category->getParentCategories() as $parent) {
    $catnames[] = $parent->getName();
}

you can add a new category attribute called page_title. Here is a tutorial on how to do it.
Then edit the template app/design/frontend/{package}/{theme}/template/catalog/category/view.phtml and replace

<h1><?php echo $_helper->categoryAttribute($_category, $_category->getName(), 'name') ?></h1>

with

<h1>
<?php if ($category->getPageTitle()):?>
    <?php echo $_helper->categoryAttribute($_category, $_category->getPageTitle(), 'page_title') ?>
<?php else: ?>
    <?php echo $_helper->categoryAttribute($_category, $_category->getName(), 'name') ?>
<?php endif;?>
</h1>

now you can fill in for your DEF category the field page title with DEF stuff and it will be displayed on the page. If you don't fill in the page title field, the category name will be used.