Extending my tour in afghanistan?

I'm nearing the end of my tour in Afghanistan but I was considering extending to stay longer. If I extended to stay longer I wanted to transfer to a unit in another area of the country which still has about 6 months left in their tour. Does anyone know how I would go about doing this? Would it be as easy as filling out a 4187 or would I need to do something else? Also would I be able to transfer to a specific battalion or only to a specific brigade? Thank you.