I don´t know if someone approached this matter or if it is intentional but regarding the early vesions of some units is that the research you need for the the tier III of the barracks is the same that replaces your units with high versions which effectively means you can never use the early version.
I know some of that units are recruitable in the castle chain but does not make sense that a tier 3 building, that unlocks early units to be in the same technology that replaces them.
Best regards