I think we should be allowed to upgrade factories before/while building an hq. This would allow you to shift to t2/t3 more fluidly/quickly. Obviously the support factories wouldn't be able to build anything until the HQ is complete.
In my humble 800 rank opinion, I don't think it would break balance too much. It would certainly help me with scrambling enough percivals to fight a Monkeylord rush, or quickly building up mmls to counter turtle. What do you guys think?