If you asked a hundred people, they would probably give you a hundred different answers. That's just to preface my suggestion since other people will probably say "that's not the best way".

I would start with a layered texture (not layered shader) plugged into a shader. Load all 12 of your textures into the layered texture, with the first one to be shown at the top, and the last one at the bottom. Then you could use set driven keys: rotation of the earth as the driver, and the alpha of each texture layer as the driven.

Halfway through your rotation animation, you will have the first 6 textures with an alpha as 0 and the next 6 with an alpha as 1.

Hope that makes sense.

Edit: Sorry, I mean the rotation of the earth around the sun, but since I see you're using motion path, hopefully you can set the driver as the distance along the path.