Go to "EDIT MODE" [Tab].
Move the "item.R" bone to the left on the X-Axis [G, X] - that's just pointing forward from the right hand
You should also rotate it 180˙ on the Y axis [R, Y, 180, Return], to prevent reaping with the back of the sabre.
Parent "item.R" bone to "hand.L": Select the two bones [Shift + Click] in this order and bind them together [Ctrl+P]. Select (Keep Offset) from the dropdown menu.
Blender will do the same simultaneously with "item.L" and "hand.R" while the (X-Axis Mirror) is checked in the {Editing Options}. So, uncheck it if you want to keep the shield in the left hand.
Go back to "POSE MODE", and make your left handed animation.
Beware, this change will only affect the animation you make after the bone swapping. Shield and Sword will jump back if you use it followed by an official animation!