I think, as long as the maintenance is updated then no problem on upgrading it. Else, u may contact your account reps or any solarwind customer service
To be thorough, your customer should upgrade from 7.X -> 8.5.1 -> 9.0/9.1.
As with any upgrade, we recommend you create a backup of your database before you start the upgrade. For more information, see the "Moving Your Orion NPM Database" technical reference.
For further assistance specific to your customer's environment, please submit a ticket.