NBU appliance upgrade and catalog

Hi, my customer has one NBU 5230 appliance which is currently running on 2.5.3 and it is configured as a master server. There are some clients which are backed up to this appliance's dedup pool. All clients are Windows clients (NBU 7.5.0.6 client). Recently they want to upgrade this appliance to 2.6.0.3. My question is:

Will the catalog in the appliance automatically upgraded during the upgrade from 2.5.3 to 2.6.0.3? If no, what addtional steps shall we do to convert the catalog? If yes, if the catalog is corrupted during the upgrade, how to restore the appliance back to its original state?

Thanks in advance.