Plugin for changing a post’s category based on it’s post date?

Don’t know of a plugin but you can use wp_schedule_single_event function.

First create a meta box that takes to values: time for removal and what category we want to set it to when removed from featured.

 /* hook meta box */
add_action("admin_init", "admin_init");

/* hook meta box function */
function admin_init(){
    add_meta_box("Featured Removal", "Featured Removal", "Featured_Removal_options", "post", "normal", "high");
}

/* display meta box */
function Featured_Removal_options() {
    global $post;
    $custom = get_post_custom($post->ID);
    echo '<input type="hidden" name="wp_meta_box_nonce" value="', wp_create_nonce('Featured Removal'), '" />';
    <?
    <table border=0>
    <tr>
        <th style="width:20%"><label for="Remove_after">Remove From Featured After:</label></th>
        <td><input type="text" name="Remove_after" id="Remove_after" value="<?php $custom['Remove_after'] ? $custom['Remove_after'] : ''; ?>"/><br/>
        Enter time in Seconds Ex: 1 Hour = 3600 Seconds , 1 Day = 86400 Seconds.
        </td>
    </tr>
    <tr>
        <th style="width:20%"><label for="Remove_after_to_cat">Remove From Featured To Category:</label></th>
        <td><input type="text" name="Remove_after_to_cat" id="Remove_after_to_cat" value="<?php $custom['Remove_after_to_cat'] ? $custom['Remove_after_to_cat'] : ''; ?>"/><br/>
        Enter the category id of the category you want to remove the post after the time has passed. if more then one separate by commas Ex: 12,13,24
        </td>
    </tr>
    </table>
<?php }

/* save meta box hook*/
add_action('save_post', 'save_Featured_Removal_options');

/* save meta box function*/
function save_Featured_Removal_options($post_id) {
    if (!wp_verify_nonce($_POST['wp_meta_box_nonce'], "Featured Removal")) {
        return $post_id;
    }
    // check autosave
    if (defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
        return $post_id;
    }
    If (isset($_POST['Remove_after']) && isset($_POST['Remove_after_to_cat'])){
        //cerate scheduled event
        $time = time() + $_POST['Remove_after'];
        wp_schedule_single_event($time, 'Clean_my_featured',$post_id);
        //save meta data
        update_post_meta($post_id, 'Remove_after', $_POST['Remove_after']);
        update_post_meta($post_id, 'Remove_after_to_cat', $_POST['Remove_after_to_cat']);
    }    
}

Now watch the save Meta Box function , if the user has enter both time for removal and category id for the new category then we set a scheduled event
with wp_schedule_single_event and hook it to “Clean_my_featured“.

So now we only need to add the action for that hook and the function for the removal itself:

 /* hook removal event function */
    add_action('Clean_my_featured','remove_post_from_featured');

// the function that removes a post form a category and sets a new one
function remove_post_from_featured($post_id) {
    $cats = get_post_meta($post_id, 'Remove_after_to_cat', true);
    wp_set_post_terms( $post_ID, $cats, 'category');
}

I have no Idea if this works but it should so just copy all of it to a plugin file or your themes functions.php file and it should work.

if not let me know.