Ship Upgrades is a system that allows to change a bought ship into another one that costs at leasts as much preserving all limited items, access passes and insurance levels. Ship upgrades can be applied to ship packages and standalone ships. Sometimes ship upgrades are referred to as Cross Chassis Upgrades (CCUs).
In the past a very complicated system of upgrades was maintained. It allowed only for upgrades between selected ships and created upgrade paths that resulted in different prices. It has changed in 2015.