There's a convenient little utility called
symlinks 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 It's no longer maintained but still available in some distributions.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) 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