Faced with a similar challenge, we opted to upgrade to Java 5 style generics only in the code that was edited for another reason. So if you have a bug to fix in DoItFast.java, update DoItFast.java to use Java 5 style generics. The areas of the code that are being edited and changed regularly will get updated quickly. After a few weeks or months of this behavior, you can reevaluate the condition of your code base.
If that doesn't get the job done fast enough for you, sometimes I'll use the slow time after lunch to just mindlessly cleanup a few classes and speed the process along.