What is the opposite of a retronym?

A retronym is the name given to an obsolete or older object to differentiate it from its newer replacement. Examples include "straight razor" (once just called "razor" until the modern razor), "analog clock," "manual transmission," etc.

What is the term for the corresponding word that shifts meaning from the retronym? (In these examples, how do "razor," "clock," and "transmission" function?)

Oxford Dictionaries lists retronym as being a compound of retro- and -onym, with retro- denoting something back in time.

By analogy, one can substitute ante- to denote something earlier in time to yield: antenym. (Not to be confused with antonym.)

As to what an antenym is, let's use the example from Oxford's definition of retronym:

A new term created from an existing word in order to distinguish the original referent of the existing word from a later one that is the product of progress or technological development (e.g. acoustic guitar for guitar).

So you can say that guitar is the antenym of the retronym acoustic guitar.