The easiest part of the transition from RHS to LHS is the flying and by the time you have finished your upgrade you will feel as though you had never been anywhere else but the LHS.
The hardest part is the mental transition from co-pilot or FO to Captain, from having little or nothing to worry about to being in total control and more importantly totally responsible. This last part usually becomes most apparent in the SIM, which is what it is for, emphasising the need for good command training.