The RootsMagic registration key can be cleared by opening RootsMagic in a main view and using the key combo Ctrl+Shift+U, selecting "Reset Registration Information", then closing the program to clear it. If you receive a message that prevents you from opening RootsMagic... You can manually clear the registration key by deleting the RootsMagicApp.xml file.
The RootsMagicApp.xml is located here.
Windows: C:\ProgramData\RootsMagic\Version 8\RootsMagicApp.xml
Mac: Public/RootsMagic/Version 8/RootsMagicApp.xml
The next time RootsMagic is opened you can select to run in Essential mode, purchase RootsMagic, or Unlock RootsMagic 8 with a registration key.