Tow bar change

I would think that it depends on the design of the towbar itself, which will be specific for the make and it may also not be possible to purchase just the swan neck portion. You will therefore only get a definitive answer to your question by contacting the manufacturer.

I've never seen a design of tow bar that would allow you to swap from a 2 bolt flange bar (which which what I assume you now have) to a swan neck tow bar. You'll need a complete new tow bar. 

If you are considering a swan neck towball because it will work with an Alko stabiliser you could just replace a flange mount towball  with a long neck flange mount ball.

Like this.

A standard towball may damage an Alko Stabiliser hitch under extreme conditions.

If you want the swan neck for aesthetic reasons you will need to replace the whole towbar.

Witter are probably the best people ask about this - they are very helpful

isn't it great when people have made suggestions to a problem & the OP never comes back to respond ..... 🤐