CT TCS, p34:
REFITTING SHIPS
_ _ Outmoded ships may be improved by refitting; obsolete systems are replaced by newer models. All refitting must be done at an A or B starport, and jumpdrives may be refitted only at A starports. Refitting involves the complete removal of an old system and the installation of a new one; for instance, if a power plant is refitted, the entire power plant is removed and a new one put in its place. Refitting takes up shipyard capacity equal to the refitting ship's tonnage.
_ _ Changes in power plant, maneuver drive, or jump drive are major changes. They cost 1.5 times the amount the new system would cost in a new ship; the time required to install major changes is one fourth the time required to build a new ship (from the construction time table).
_ _ Changes in any other ship component are minor changes. They cost 1.1 times the cost of the system in a new ship and take one tenth the time required for new ship construction.
_ _ Refitting is subject to the same time modifiers and weekly costs as in the ship-building rule. If several ships of a class are being refitted the same way, all ships after the first receive the time benefit. Work may proceed concurrently: if several ship systems are being replaced, the refit takes only the time required for the longest one.
_ _ The degree to which a ship may be changed is limited. Power plant, Mdrive, Jdrive, and spinal mount weapons may not be increased in tonnage. There may be no additional launch facilities built (although they may be removed). Armor and configuration may not be changed. The number and size of weapons bays may not be changed.