The usual things that cause the PCID to change are hard disk replacement, or re-partition into different drive letters, or major a OS update. I suppose that it is possible that if the CMOS battery died, the BIOS configuration could change in some strange way to trigger it.
Try checking the PCID again after the machine has been off for a few hours. If it comes up with the same new PCID, I would recommend getting a new license to match and move on with your life, you may never know why it happened. The Mach4 license procedure will not penalize you for doing this.