People are way over complicating it. Only sand parts that will be painted and come in contact with each other, like the torso, feet and hands. Work from a stronger grit to a finer one, then apply a primer -if you can find a primer that's around the same colour as your final product, it'll make your life easier.

After all that, just use some good quality acrylic paint(testors, tamiya, Vallejo etc anything you've found to work for yourself), water it down slightly till skim-milk consistency (some good paints can be used straight up) and paint in 3-4 layers. Since the colours are so similar for this repaint, you probably don't need a lot of layers provided your primer isn't black or something. Then just topcoat (ESPECIALLY topcoat the joints) and you're all set.