Nothing to do with ability in my airline. (well when I say ability it assumes that when you have command in the company you have proven your ability) It's all got to do with seniority. Each year you put a bid in for what you want. If you're senior enough in the left seat of shorthaul you simply do the conversion course from one aircraft to the other. You certainly don't go back into the right seat again just because of the type of aircraft. Taking captains out of the equation, when the airline had turboprops we actually used to have some FO's going from turboprop rightseat to longhaul jet without having any previous jet experience. It never seemed to cause any problems.
It's quite funny actually as in the past few years it is not always one way traffic. Some shorthaul captains are now putting a bid in for longhaul, doing it for a year and then coming back. Believe me it's not all it's cracked up to be in my experience. Long hours of boredom followed by rapidly reducing rest periods. Certainly hasn't the excitment factor of years ago.