Given that the amount of ne wunits surely will not fit on the tactical units list in Galactic Conquest, we have several ways we can code the units to only appear in their tech levels.
First way: write events so that old units are switched with new ones (much like Z95s are replaced by XWings)
Second way: Tech Levels make more space buildings available, which are used to build newer units (instead of building units directly from the space station)
Third way: for every faction, make certain units only available on certain worlds (so you get a mix of techs all the time, but different units buildable on different planets)
As an example:
First: Venators replaced with Acclamators, replace by Victories, replaced by ISD's
Second: Tech 1: Basic Fighter factory (V-Wing, ARC-170) Tech 2: Fighter Factory (TIE Bomber, TIE FIghter) etc.
Third: Hypori: Venators, IPV-1, V-wings Courkrus: Tartan Cruisers, Acclamators, TIE Interceptors (etc)
Basically, I am asking the community which method they would prefer us to code. If you still have questions, I could write a full diagram of each one for you (just give me a bit)