To use a new System ID you must delete the existing one. To delete the System ID for Sibelius 7.1 (or later) you need to trash the following folder:
Windows
C:\ProgramData\Avid\Licenses\ delete folder: ‘Sibelius’
NOTE: ProgramData is a “hidden” folder so make sure hidden files and folders are visible under Tools > Folder Options > View (You can also get to Folder Options from Start>Control Panel)
Mac OSX
MacHD > Library > Application Support > Avid > Licenses > delete folder: ‘Sibelius’