2 of 2 symlinks is maintained now (thanks Evgeny Vereshchagin)

There's a convenient little utility called symlinks (originally by Mark Lords, now maintained by J. Brandt Buckley), present in many Linux distributions.

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