Downgrading may not be a good choice, but from what I can see JetPack3.0 (which installs L4T R24.2.3?) would be Ubuntu 14.04 and newer would be Ubuntu 16.04 (be careful when searching for Ubuntu version to view whether the version is for host or for Jetson).
You can see the L4T 24.2.3 page here:
JetPack3.0 is here, but you need to be logged in to see the download button:
Someone else may want to add information on ROS versions, in which case you might need to add more information about the need to downgrade ROS.