There are a lot of changes under the covers with Project Server 2013 including a consolidation of databases, retirement of BCM, stopping in place upgrade support (nobody really did that did they?) and the underlying changes to SharePoint that means upgrading a PS instance is not as simple as it once was. To address this, the product team have released a fantastic poster outlining how to upgrade from Project Server 2010 to Project Server 2013. The poster is for the preview only at the moment and may change before the RTM, but it’s handy enough to help you get an evaluation environment up and running containing your migrated data.
The Project Server 2013 upgrade process can be downloaded as a Visio diagram or as a PDF document. Also, keep an eye on http://technet.microsoft.com/en-us/library/cc197692(v=office.15) for details of all Recently published content for the Project Server 2013 Preview.