Why does a unit lose half its readiness for changing its HQ? I understand the disruption to its supply chain, but that seems a bit harsh, especially if the unit didn't move, and its new HQ is only a couple of hexes away.
The official explanation is it causes a disruption to its chain of command.
There is probably a game technical reason too. I am just not sure what it is. I could imagine someone however switching units to HQs with better Staff numbers to ensure a better combat result, if there wasn't a hit to readiness.
It doesn't have to be that way though. There are scenarios and mods out there where the penalty is less, or completely gone.