–Remove the Identities folder from the Registry

Office 2016:
HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Common\Identity\Identities and delete the “Identities” folder
Office 2015:
HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Common\Identity\Identities and delete the “Identities” folder

–Run the scripts mentioned in below by running the command prompt as administrator

Office 2016:
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cscript.exe “%ProgramFiles(x86)%\Microsoft Office\Office16\ospp.vbs” /dstatus

cscript.exe “%ProgramFiles(x86)%\Microsoft Office\Office16\ospp.vbs” /unpkey:XXXXX

Found on Spiceworks: https://community.spiceworks.com/topic/1790625-how-to-change-office-365-desktop-install-licensee?utm_source=copy_paste&utm_campaign=growth