From some of the thing said, or implied, it seems you need not retrain when you get a tech upgrade any longer.

Armor upgrades don’t seem to be that big a deal this time through…they don’t wear it any more.

I don’t know or have not heard about weapons upgrades, specifically.

But the fewer upgrades your troops need the better. Especially when you often pay for them and nothing happens, and you need to do it again and again…