No need to reimage, the image is the same for both languages.
You only have to reimage if the version is not up-to-date and then it's just using the roboRIO Imaging Tool as usual.
You do need to install Java 8 on the roboRIO
Installing Java 8 on the roboRIO using the FRC roboRIO Java Installer (Java only)
The roboRIO boots the last language user program placed on it.
The only special procedure is for going from C++/Java back to LabVIEW, not the other way around, because the C++/Java programs don't give up easily.