While reading the tech tree files, I see that I misunderstood what the "End_Tech_N" events do, which is not advance the tech level. Actually, there are no events that advance the tech level, so I added my own, for example:
That is, when the rebels finish researching all itens from era 1, they advance to tech 2, which upgrades all complements and garrisons as well (using <Starting_Spawned_Units_Tech_?> tags). I like to think that, as their technology advances, older units are phased out and are used in guard duties, while the newer models spearhead assaults. So, Very late in the game, fully upgraded Y-Wings can be found in Golans and Carriers, while B-Wings and K-Wings are strong enough to operate in carrier-independent squadrons.