You could try the following plugin: http://wordpress.org/plugins/ambrosite-nextprevious-post-link-plus/ It is very customizable in terms of link options and should, as far as I can see, do the job for you. I have very good experience using it up to WP 3.5.
Url PrevPost after migration
If you are using a custom query you will have to build your own navigation.
See the codex. The first parameter is format, the second is link text. <?php previous_post_link(‘<strong>%link</strong>’, ‘«’); ?> <?php next_post_link(‘<strong>%link</strong>’, ‘»’); ?>
Maybe this function is better to use get_adjacent_post. I have not tested it yet but it seems to more flexible than the_post_navigation() function. FYI, the_post_navigation() uses this function internally to retrieve the next and previous posts.