Project Professional 2007 and 2010 can co-exist on the same client machine, and while Microsoft recommends ditching 2007 as soon as you can after a 2010 upgrade, this may be the ideal situation to allow users to test a 2010 SharePoint/Project Server upgrade while still working with the 2007 version in the production environment.
Once 2010 Project Pro is installed, however, it becomes the default version launched from hyperlinks or files – whether intended or not. This may mean that when a user clicks a link in a 2007 Project Web Access (PWA) site (in our case, the link to edit the Enterprise Calendar), Project Pro 2010 will launch, and the user will receive an error message that the file cannot be opened because Project Pro 2007 is not installed on the client machine, when in fact it is installed.
To resolve this issue during the transition period, users should open the version of Project Professional they intend to use prior to clicking the SharePoint hyperlink or the Project file on the network drive. The system will look for an open version of Project Pro before opening a new instance of the default version.