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