Is there a possibility to mount a 2014/2015 Croozer Cargo trailer hitch onto a 2012/2013 model? I've got a 2012/2013 Croozer Cargo trailer and its trailer hitch is very clunky. It's also difficult to couple and uncouple and I've hurt myself multiple times on the quadratic bolt sticking out at height of bicycle's rear axle. Additionally the part mounted to the bicycle makes it wider which causes issue when putting it in the boot of my car. (The bicycle is a Brompton folding bike and hence the size of the hitch matters, at least if it sticks out on the side like 5cm.)

Recently I saw new Croozer Cargo models at a dealer and I noticed that they now have a completely different trailer hitch system. Way more compact, less clunky, no more sharp 90° edges and easier to couple.

According to the dealer there's no chance of getting the new hitch system combined with my previous generation trailer—but this didn't seem a technical founded answer.

So anyone here knows if there's a possibility of retro-fitting my previous generation Croozer Cargo trailer with the current generation Croozer trailer hitch?
Maybe.  It depends on differences beyond the hitch system for the trailer.  Best bet is to find the model you want to upgrade to and take measurements and compare.  

Typically you will find that the trailer tongue is made of plain tubing (be it square or round).  I doubt that this will change much between model lines and years.  What will change is probably the various bends in it to give the tire some clearance.  The important part is that it is the same size tube on the inside (or really close).  The hitch parts are then inserted into the tube and secured with a nut and bolt.  If the inner diameter is good and the securing bolt is in the right place, make the swap.  

I have done such swaps on both of my trailers.  My kid trailer is a Pacific Voyager 2 and the other is a generic cargo trailer (happens to be a Asom Wonderer, but other sell it as well).  Both trailers had a spring that came out of the tongue into the quick release assembly which then attached to the component that is mounted on the bike.  It ends up that both trailers have tubing that is the right size for the Burley systems so it was a quick swap.  All I did was undo the bolt, slide out the spring, then add the flex connector and bolt.  Here is a picture of the setup from my cargo trailer: