Hi Keith,
I tested the renaming of the image files while in the Arrangement sort mode and the pmarrangement.dat file is updated as soon as the rename operation is done. When I open the pmarrangement.dat file in a text editor app, I can see the new filenames from the rename operation. Have you tried deleting the pmarrangement.dat file and then let PM create a new one? Quit Photo Mechanic and then delete the .pmarrangement.dat file using the Explorer window. Start Photo Mechanic and open the same folder of images. Use the Make Arrangement option from the View menu. Now can you rename the images in the Arrangement sort mode and have Photo Mechanic update the pmarrangement.dat file?
Best regards,
--Bob