OS X Mountain Lion: Change resolution automatically when on battery
Your best bet would be to use ControlPlane (free) to detect when the power source has switched, and then fire off a shell script to change the resolution.

There's no built-in command line utility for changing the screen resolution, but someone came up with one that works well. You can download an installer here.

Then it's just a simple matter of setting ControlPlane to run screenresolution set 1440x900 or 1650x1050 as appropriate. Note that you may want to experiment with the screenresolution get command first to double check how it handles retina resolutions.