well ... oh a hypothetical standpoint you can download only the base and install it and be semi happy
BUT what if say ... your current Kmail (which i imagine is part of networks) needs to access an interface that has been changed? You might find yourself with some flat out incompatibilities, or you might be lucky
You don't need ot upgrade everything, in theory you should be fine if you're running 3.2 atm, and are just upgrading some parts but ideally you want it all up to date.
Also depending on which one you download might need to resolve dependencies, plus on RPM based linux systems the order of installation matters ...
I'd say get them all, except the i18n packages, if you don't compile from source you probably won't need the development packages. but if i was you i'd install all to retain stability, be up to date, and pre-empt the problems of installing subsequent packages, that do require some dependency you sidestepped now.