According to the new manual its included (For the same weapons tree atleast). Does it work ingame atm?
Weapons Retrofitting
Design a ship with particular weapons means a whole sub-system of control
and power has to be put into place. These power systems can be modified to
accommodate similar weapons, but they cannot be changed entirely to take on
a different weapons family. Meaning, if you build a ship with energy weapons, it
is possible to upgrade it later to better energy weapons. But you can’t decide to
change from energy weapons to missile turrets later – different sub-systems are
required. You can upgrade your fleets in the Fleet Manager.