Upgrade Spring Java WAR project to JDK 1.8 from JDK 1.6

I recently inherited an old Spring 3 Java WAR project that's been aging unattended in the corner for many years, happily running in production. Now due to infrastructure requirements we need to upgrade it to JDK 1.8 from JDK 1.6. With my very rudimentary Java skills (i'm a windows developer), I think I have it mostly working - but haven't been able to figure out how to get Spring Roo properly updated. Googling hasn't helped, nor has searching internally for someone with a proper skillset. So.. the goal is to get this code base to build against JDK 1.8, making whatever dependency updates are needed. The stack/technology includes Spring Framework, AspectJ, Spring Roo, and Maven. My hope is that this is pretty straightforward for someone with decent experience in the toolset (if it helps, the app is not very big nor mission critical - just an internal tool customer support representatives use). Long term we might rewrite this in C# so that our team can better maintain it, so I just need a band aid for now.. the smallest change that gets it to build, even if it's not the modern way of doing things (ie, not looking for an overhaul into the most modern Spring 5, etc).