Most upgrades are available but disabled due to requirements. Check out the requirements for each of the upgrades that you'd like to have available and work from there.
I think that this thread has the information that you need.
I am working on a map that has an APC as a garrisonable unit. The APC has a gun on it, and I'd like to be able to display the damage from the gun and the armor of the APC even while units are garrisoned inside of it. Is there any way to do this?