If you're playing Crusade, you don't upgrade with constructors anymore -- you simply click the upgrade you want and it will upgrade providing you have the resources. Tool tip should tell you if you're missing any.

Someone can correct me if I'm wrong, but most of the ring paths are mutually exclusive. For, eg: If you set up a starbase and add a mining ring to it, you cannot then add an economic ring or a cultural ring. Some upgrades span all of the main rings, though like weapons systems, scanners etc. Also, I think archaeology labs can be added to any starbase too.

It was only possible through a mod.

This is true and has been throughout GC3, and was even true in GC2.

It is possible if you're willing to mod it, but with Crusade I have yet to build anything other than a mining starbase, so I haven't missed the old mod that let you add mining to any other type starbase.