So - I trust you pay for the upgrade during the Purchase & Repair Units phase, and then the upgrade actually takes place during the Mobilize New Units phase. Is this correct?
Yes.
If this is correct, then I assume that the newly upgraded IC (now a Major Industrial Complex) cannot begin to mobilize its full quota of 10 units until the owning player’s next turn. Is that right? It can still only build 3 units during the turn in which it was upgraded. Please advise if I am wrong.
You are correct.