We present measurements of the magnetic penetration depth, lambda(-2)(T) , in Pr2-xCe xCuO4-y and La2-xCe xCuO4-y films at three Ce doping levels, x , near optimal. Optimal and overdoped films are qualitatively and quantitatively different from underdoped films. For example, lambda(-2)(0) decreases rapidly with underdoping but is roughly constant above optimal doping. Also, lambda(-2)(T) at low T is exponential at optimal and overdoping but is quadratic at underdoping. In light of other studies that suggest both d - and s -wave pairing symmetry in nominal optimally doped samples, our results are evidence for a transition from d - to s -wave pairing near optimal doping.