It is not too difficult to replace the knuckle spring. The hardest part is carefully grinding off either the head or the bottom of the rivet so it can be pressed out without damaging the coupler.
The tricky part comes in reassembling the knuckle with a new rivet and spring. You need three hands to hold the knuckle in proper position, get the spring oriented properly, and then slide the rivet through everything. Squeeze the very bottom of the rivet to hold it in place with pliers, making sure the knuckle is free to open and close. Patience is needed.
Replacing the spring is only necessary if you wish the coupler to open on an uncoupling track.