I went from flying fast small things to flying rotary wobbly things to flying a PA28 for reasons only know to the CAA and the then National requirements and my next aircraft after the PA28 was an A320.
Is it difficult? In my opinion no. The conversion course (provided by the company way back when :-) ) was excellent so the line training was great fun.
At the end of the day it's basically an aircraft, same as the PA28 but with more weight, thrust and inertia. Getting used to the side stick control laws back then was easy for me as big helicopters fly in a similar manner (roll rate demand).
What does take some time is getting used to the planning and conduct of the flight, the fore thought, spacial awareness and think ahead required to operate such aircraft in and out of busy airports and airspace. Also ensuring that you listen and learn as, as has been posted previously, at that point you're not bringing much experience with you in your case, the TC will be working his/her 'metaphorical' nuts off (whilst trying not to show it obviously!).
It all comes with time.
