There's a convenient little utility called [`symlinks`](http://linuxcommand.org/man_pages/symlinks8.html) by Mark Lords. It's no longer maintained but still available in some distributions. If yours doesn't have it, you can grab the source (e.g. from [Ubuntu](http://archive.ubuntu.com/ubuntu/pool/universe/s/symlinks/symlinks_1.2.orig.tar.gz)) and compile it.

To convert all the absolute symbolic links in a directory to be relative:

    symlinks -c /path/to/directory

Unfortunately there's no option to convert a single link, you'd have to move it to its own directory first.

    mkdir ../tmp
    mv link-to-relativize ../tmp
    symlinks ../tmp
    mv ../tmp/* .
    rmdir ../tmp