If you copied your virtual machine and need to change its UUID, please follow these steps:
1. Remove the virtual machine from the Virtual Machines list. Please drag the virtual machine out of the list or right-click on it and choose Remove --> Keep file.
2. Locate the PVM bundle of the unregistered virtual machine in the Finder and double click on it. The default path to the virtual machine's files is your Home folder/Documents/Parallels.
3. You will be prompted to choose whether the virtual machine was copied or moved. Please select "Copied VM". This approach should generate a new UUID for the copied VM.
The same procedure can be accomplished using the Terminal application.