Additional ways to transform to Graphics and post-process the Graph output:
Show[Graph[{Labeled[a->b,"A"],Labeled[a->b,"B"]}]]/.
Text["A",{x_,y_/; (y<0.)},z___]:>Text["B",{x,y},z]
Or
Show[Graph[{Labeled[a->b,"A"],Labeled[b->a,"B"]}]]/.
BezierCurve[{{-1.,0.},m_,y_}]:>BezierCurve[{{1.,0.},m,{-1.,0.}}]
(* same picture *)