Final answer:
The Vm change when P Na is altered would depend on the specific electrochemical context of the cell and the role of Na+ ions within it, which is not provided here. Additional information specific to the cell's environment and reactions is necessary to determine the effect on Vm.
Step-by-step explanation:
When considering changes in the galvanic cell voltage (Vm), it's important to understand the role of ion permeability, in this case, the permeability of sodium ions (Na+). Changing the permeability of Na+ (P Na) to 0.001 would affect the cell's potential. However, the information provided does not directly correlate to changes in permeability but rather to various scenarios in electrochemistry involving Na+ ions such as in a concentration cell with Pb²+ ions and the Nernst equation, the vapor pressure decrease caused by NaCl in seawater, and the reduction of oxygen to water in cells exposed to atmospheric conditions. To accurately answer how Vm changes with a changed P Na, additional context specific to the type of cell and the ions involved is required.