HG'2022, p72:
There are two different types of refit that can be used. Major refits cover changes in power plant, manoeuvre or jump drive, as well as changes to spinal mounts or launch facilities (such as launch tubes). Removing these components costs 0.5 times the cost of the original system, while removing them and then installing new ones costs 1.5 times the cost of the new system. The time this takes is one quarter of the time required to build a new ship of the same size as described in Construction Times on page 8.
Minor refits are changes to any other components aboard the ship, such as weapon mounts or staterooms. Removing these components costs 0.1 times the cost of the original system, while removing them and installing new ones costs 1.1 times the cost of the new system. This takes one tenth of the time required to build a new ship of the same size. Armour and other parts of the ship integral to the hull (such as configuration or reinforced structure) cannot be changed under any refit. Those items covered under a major refit cannot be increased in size but they can be reduced. Other components can be increased in size if there is tonnage available.