Replace Link Text

The tool I would normally use for this is the interconnect search & replace tool: http://interconnectit.com/products/search-and-replace-for-wordpress-databases/

If the links are only in post/page content, so unlikely to be contained within serialized data in the database, you can use a simple search & replace plugin. There are quite a few available, so you should be able to find one you like: http://wordpress.org/plugins/search.php?q=search+replace