12 hours ago, iamfanboy said:This.
A basic chassis, in military terms, just dictates general terms like the displacement of a hull - and because building the hull is a huge initial cost in both time, money, and materiel, it's often more cost-efficient to upgrade them when new technology comes out rather than replace them entirely. It cost half a million each to replace the electronics in an F/A-22 a decade ago, for example, but $95 million to buy an F-35A - buying one F35A costs almost as much as replacing electronics in the entire FLEET of F/A-22s that the USAF runs. Military craft are often designed with the upgrade cycle in mind, too; components are designed to be easily removed and installing more modern ones will often take about as long as replacing an old one.
So in real terms, ISD-Is would go into drydock and come out six months later as ISD-IIs, rather than spending a year building a new ISD-II from the ground up. Times are completely guessed at in nucanon, mind you.
the EU's version of this was with Mandators. In The Essential Guide to Warfare , we find that the Republic upgraded some original Mandators to Mandator-II standard, in addition to building all-new Mandator-IIs.