There's a reason new Gen II plants cannot be built, and all the regulations and safety reports in the world will not fix the fundamental design flaw of these plants.
We can mitigate and make meltdown less likely, we can't eliminate it without replacing the plants all together.
That's a bit of an impossible ask.
To give you a comparison with airplanes, F16 aren't "upgraded" to F35s. But there is an upgrade process, and F16s today are vastly different from F16s as they were in 1978.
Likewise for nuclear plants, reviews are done following incidents and new discoveries, and overhauls are done, both in terms of process and material changes. Gen2 plants aren't the same as they were when they were built.