almost right, but not quite.
it partly depends on what trigger your replacing. Because the hammer lug position in relation to the trigger sear may also need adjusting. as it did when i fitted a blade to jeevus's outcast cocker.
its not a bolt on job.
Trick would be to speak to someone who knows their shiznit about cockers, and get them to do it for you. be it a friend, or a shop. Its a quick job IF you know what your doing.
if you feel adventurous you could read up on
Ravi's Site
his info has helped me out with this stuff many times.
But officially, yes it will need retiming.
hope that helps