I replaced a baseboard heating boiler (oil/gas) the new zones at the expansion tank etc, were changed to 3/4 inch. They go into my house baseboard which is 1/2 inch. The second floor is a problem with water noise and popping. The pressure level was set at old level. Can I reduce the pressure below the 12 psi?

I would not decrease the pressure to the 2nd floor because that is not the problem. You have air in the system and it needs to be removed. Bleed the system especially the top floor to cure the problem.