Zencart Upgrade and add new modules

I´m working on a Zencart 1.3.6 version and i want to upgrade to the new one 1.3.8 without lossing any of my actual modules (unless the actualization require it). Also want to add 3 forms of shipping, they should be "COD" (with extra charge, "Estafeta" (kind of Fedex or UPS), and "postal Service". Actually working with a country with 2 zones (Mexico: D.F and Rest) i want to add all "rest" cities so only can be able to select "cod" service if they live in D.F. or Mexico and no other city or country. Same Estafeta, this should be selectable only for shipping orders inside of Mexico