You'll need to ensure that the customer has a UCSS contract in place for all of their mailboxes; the quantities should match between configured mailboxes and the quantity shown on the UCSS agreement. Once that is done you can log into PUT and place an upgrade order. You'll want to migrate them to Unity Connection; traditional Unity is dead.
If the customer does not have an active UCSS contract you'll need to provide an a-la-carte upgrade quote. The Ordering Guides will walk you through that.
When it comes to the actual upgarde make sure you read the Release Notes, Supported Platforms list (you'll almost certainly need to increase HD/memory if not a new server), the Install Guide, COBRAS help, and Unified Messaging guide (Single Inbox in Connection is not identical to Unified Messaging in Unity) amoung other product documentation.
Finally, if you do purchase a new server you'll need to review the DocWiki for all of the UC on UCS guidelines.
