Not the OP, and our stacks/reasons are probably different, but for perspective, I work on a large Canadian Federal Government project, and a minor change in Java version on severs for our legacy application, is probably... 1-2 months elapsed time, minimum. And that's if we get the change approved. Mostly it's the slow and manual change management practices, slow and manual approval framework, slow and manual testing approach, slow and manual implementation procedures, and other things that are slow and manual.

Maven, Hibernate, Jersey, Jettison/Jackson. All developed on Java 7 I believe, so the jump is rather large.