I am not a naval architect so I really don't know how much redesign work would be required when converting from reactor F to reactor I. Dimensions and weight are the main considerations but perhaps there is a lot of other stuff as well.
Different reactors have different types of fuel, steam generators, vessels, core, primary and secondary circuits...
Two models of the same generation and with the same tech (for example two pressurized water 3rd gen reactors) can be extremely different, depending on the design you can have one that is double the size of the other with the same output in terms of MWs... or one that has 3 steam generators and one that has 5...
A reactor could need refuel after 10-20 years, another might be built completely closed and at the end of its life its just unusable...
