You should be able to replace most PLCs in AS 4.x. If you’re running ARsim and trying to replace the PLC, that may be the issue, as you can’t replace CPUs while in simulation.
If you could supply us with some screenshots, we can dive a bit deeper into why the “replace hardware modules” is greyed out.
Just in case, instructions on how to replace hardware modules can be found in the help explorer linked below.
Replace hardware module