You can check what needs to be fixed with them now (if anything) and do the renovations to keep them working. As long as the basic design is still considered save today, and as long as maintenance and running costs are well below the revenue you make.

The biggest expense in nuclear power is building them. And a really big part of that exploding cost is in all the dark rituals you have to engage in to placate public opinion. (Like excessively long safety reviews and whatnot.)

If you take an existing nuclear reactor, the status quo works in your favour. Even in the unlikely scenario where your renovation essentially replaces the whole thing (so from an engineering point of view, you might as well build it from scratch), renovation might still be the wise choice exactly because of status quo bias in the population.