So, the Empire can still produce the same units as Tech 3 on Tech 4? Or is it that you keep the specific unit you produced?
They can't continue to produce the units. The question and answer was about whether the units you already have made will stay with you, and the answer to that is yes. We won't take away anyone's Tectors when Isard dies, Sovereigns when Palpatine dies, or anything else when somebody else dies. You just can't make more of them.
(This question is similar.) Also, is this talk of "new classes" about replacing the specific units that have already been produced? Or just upgrading the ones that you have an option to produce?
As you go along in tech levels, you will lose the ability to produce certain units (as everyone already knows. Sometimes this is because of a lack of resources, facilities and expertise canonically, and sometimes because a new class has been created that fills the same role, and was designed in universe to replace an older class. As I said, an example of this is that the New Republic designed the Corona to replace the Nebulon-B. In the mod this means you will no longer be able to produce Nebulon-Bs at a certain tech level, though you will gain the ability to build Coronas. What I'm saying here is that we may make it so that at the tech level when the Nebulon-B gets replaced by the Corona in the build options, all of the ones you already have built in the galaxy get upgraded to Coronas
Is it a bird? 
No, its Arb and the TR Team! 