You can also do it in WMM 6.0 using Rehan's ShaderTFX which has a Green Chroma transition.
But to do it correctly, you really need software that supports multiple video tracks. Movie Maker has never supported more than one video track, nor do I expect it to add that feature anytime soon. Remember, the last feature update to Movie Maker 2012 was in 2012, so its unlikely to be changed going forward.
For an inexpensive alternative, look at Nero Video 2015. It can read Movie Maker project files and has a Chroma Key effect that can be applied to make the green screen turn transparent. Then put a different clip (video or photo) on the track below it.
I think you can also do it with DaVinci Resolve Lite which is free! But it is a little more complicated so there is a trade-off.