Few issues after changed Office 365 subscription from E3 to Business Premium

After changed subscription from E3 - Office 2016 to Business Premium - Office 2013, I ran into all kinds of problems. 

The first issue is the Office apps such as Word shows expired ProPlus license has expired even ProPlus has been uninstalled.

The 2nd issue is after the license issue has been fixed, I could not save files to SharePoint or Onedrive for Business in Word or Excel. It would keep prompting for login even I typed in correct credential.

Solution to 1st issue:

1. Run cmd as the admin

2. CD C:\Program Files (x86)\Microsoft Office\Office15 (Office 16 for Office 2016) or CD C:\Program Files\Microsoft Office\Office15  and press Enter.

3. Run cscript ospp.vbs /dstatus and press Enter (copy the last five digits of the product key for Office 16O365ProPlus).

4. Run cscript ospp.vbs /unpkey:xxxxx

Solution to 2nd issue:

Make sure "Modern Authentication for Office 2013" is enabled (Office 2016 is enabled automatically).

Check the registry and make sure those two keys below present.

Registry key	Type	Value
HKCU\SOFTWARE\Microsoft\Office\15.0\Common\Identity\EnableADAL	REG_DWORD	1
HKCU\SOFTWARE\Microsoft\Office\15.0\Common\Identity\Version	REG_DWORD	1

https://docs.microsoft.com/en-us/office365/admin/security-and-compliance/enable-modern-authentication?view=o365-worldwide