Go to the game install folder, APConfig\Config\DefaultInput.
Beneath 'General Bindings', replace the 4 lines with:
+Bindings=(Name="MoveForward",Command="Axis aBaseY Speed=0.2")
+Bindings=(Name="MoveBackward",Command="Axis aBaseY Speed=-0.2")
+Bindings=(Name="StrafeLeft",Command="Axis aStrafe Speed=-0.2")
+Bindings=(Name="StrafeRight",Command="Axis aStrafe Speed=+0.2")
And under 'Player Movement', add these lines:
+Bindings=(Name="W",Command="Axis aBaseY Speed=1.0",Control=False,Shift=False,Alt=False)
+Bindings=(Name="S",Command="Axis aBaseY Speed=-1.0",Control=False,Shift=False,Alt=False)
+Bindings=(Name="A",Command="Axis aStrafe Speed=-1.0",Control=False,Shift=False,Alt=False)
+Bindings=(Name="D",Command="Axis aStrafe Speed=1.0",Control=False,Shift=False,Alt=False)
+Bindings=(Name="W",Command="Axis aBaseY Speed=1.0",Control=False,Shift=False,Alt=True)
+Bindings=(Name="S",Command="Axis aBaseY Speed=-1.0",Control=False,Shift=False,Alt=True)
+Bindings=(Name="A",Command="Axis aStrafe Speed=-1.0",Control=False,Shift=False,Alt=True)
+Bindings=(Name="D",Command="Axis aStrafe Speed=1.0",Control=False,Shift=False,Alt=True)