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)