My main questions would be: why?

The CX650 is a perfectly acceptable PSU. If it's doing the job of delivery power to your system then it's doing its job. If you're unhappy with how much noise it makes, or have plans to upgrade your CPU or GPU, or are otherwise having power related issues that you've traced back to the PSU, then replacing it might make sense. But there's nothing really inherently worse about non-modular PSUs than modular ones apart from easier cable management. And the fact that modularity is generally included on higher end PSUs, but it's the fact they're higher end PSUs that's the big benefit, not the fact they're modular.

For the cooler, 120mm AIOs are kind of trash as they're generally more expensive than tower coolers with very little benefit. That said, the 8400F doesn't need much cooling, so even a terrible 120mm AIO should be more than sufficient. If it's not cooling the CPU properly, or you don't like how it looks. Sure, replace it. Any single or dual tower cooler with 120mm fans should work well enough.