But you can do that for the Spec you chose with a Tech Lab if you rebuild it.
rulebook page 12 wrote:
At the tech II level, you have to specialize. When you start building your tech II building, declare which spec it’s going to be. You have a mini-card for each spec, and you’ll place the mini-card of the spec you want to build on your base. Your tech II (and then later your tech III) buildings are tied to that spec and they can only produce units, upgrades, and buildings
that match that spec. Once you make this choice, it’s locked in for the rest of the game.
See also Official Rulings Doc, point 88