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.