From what research I've done, it seems safe to upgrade (via solution deployment) previously associated, deployed, and running SharePoint 2007 Custom Workflows (developed in Visual Studio) that are In Progress. The two key areas why I think this works is to do with the content that SharePoint hydrates each time a workflow moves into and out of memory. In old-fashioned programmer speak, the program section and data section are saved with each instance of a workflow. In current lingo, I understand Extensible Application Markup Language (XAML) is used to represent SharePoint workflow objects in XML thereby allowing the instances of all the data objects and the program code to be streamed into and out of memory in perpetuity. If you think about how challenging this is to accomplish, you'll understand why I really respect this aspect of SharePoint, and how it adds to the awesomeness of skill displayed by their development team. Caveat emptor: dehydration and r...