What's the best way to link to work items in TFS 2008 from TFS 2012?

Using Microsoft Team Foundation Server (TFS), what's the best way to link to link a work item in TFS 2012 to an older work item? Our older work items use modified templates with rules and other things associated, which I'm told means we can't just "import" or up-convert them easily. So if we can't do that, is there any way to maintain a reference to a work item created in TFS 2008 through a link or something? The best answer/solution would preferably allow users to view the 2008 work item, but I don't know if that's possible.