For starters, all the old designs are built with analog automation which is no longer available, so at the very minimum, you need redesign the automation system

For starters, all the old designs are built with analog automation which is no longer available, so at the very minimum, you need redesign the automation system. Keep in mind, every component you install will need to be certified and tracked from raw materials up to follow stated processes, which need to be nuclear safety certified too. And you need to have two separate families of parts so the requirement of diversity in implementation is fulfilled, to avoid coinciding typical failures. At least now that EPR projects are close to completion, there's now a precedent that it is even possible to implement digital automation in accordance to western nuclear safety standards. Also the mechanical safety standards are way stricter now, so you can't really just take an old design and add digital bus automation. You need more redundant safety systems, corium catcher and stuff. Not so easy.