@sean1 

As you mentioned that .  "This is a new installation and since Ambari was never upgraded,"  ....  "there are no database back up files"

1. If this a new installation then what is the reason for downgrading ambari? 

    If ambari was never upgraded  (as it is a new installation) then what is the need to downgrade it ?

2. If you do not have the Ambari DB backup then you can not recover the cluster installation even if you installed the Ambari 2.6.2  (So backup is must)