• change - generic event triggered for any change on the link
• change:source - triggered when the link changes its source
• change:target - triggered when the link changes its target
• change:attrs - triggered when the link changes its attributes
• change:smooth - (deprecated) triggered when the link toggled interpolation
• change:manhattan - (deprecated) triggered when the link toggled orthogonal routing
• change:vertices - triggered when the link changes its vertices array
• change:z - triggered when the link is moved in the z-level (toFront and toBack)
• transition:start - triggered when a transition starts.
• transition:end - triggered when a transition ends.

link.on('change:source', function() { alert('source of the link changed') })