The SharePoint 2010 Workflow platform has been carried forward to SharePoint Server 2013. SharePoint designer 2010 workflows will be upgraded as it is in SharePoint 2013 but will run in SharePoint 2010 workflow platform. All of your workflows that were built by using SharePoint Server 2010 will continue to work in SharePoint Server 2013. There are two platforms (2010 and 2013) available to choose from when you build a workflow in SharePoint Server 2013 provided you had installed and configured Workflow Manager.
If you want your 2010 SPD workflows to run in 2013 platform, then you have to recreate the workflow as the assemblies and runtime are different now in SP2013. Also, there is a change from XOML to XAML format. The activities which are implementations of activity classes, are implemented declaratively by using XAML.
The other option to run SharePoint 2010 workflows on SharePoint 2013 platform is to use SharePoint workflow Interop which enables SharePoint 2010 workflows (which are built on Windows Workflow Foundation 3) to be called from SharePoint 2013 workflows (which are based on Windows Workflow Foundation 4) . This allows you to execute 2010 workflows from within 2013 workflows. Here is the Microsoft SP Designer team blog about triggering a SharePoint 2010 workflow from a SharePoint 2013 workflow http://blogs.msdn.com/b/sharepointdesigner/archive/2012/08/18/how-to-trigger-a-sharepoint-2010-workflow-from-a-sharepoint-2013-workflow.aspx