You get the following error when trying to change the vCenter root password
Password has been already used. Choose another.
passwd: Authentication token manipulation error
passwd: password unchanged
ssh to vcenter and login as root
vi /etc/pam.d/system-password
change remember from 5 to 0
Save
i is to edit in vi
esc and wq! to save
Then try again to change the password 'passwd'
change remember back to 5 once completed 
Other helpful commands
Never expire
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chage -M -1 root;
Try to clear the password history
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echo""> /etc/security/opasswd