It's actually pretty easy.
Let's say we have this in our JavaVirtualMachines folder:
jdk1.7.0_51.jdk
jdk1.8.0.jdk
Imagine that 1.8 is our default, then we just add a new folder (for example 'old') and move the default jdk folder to that new folder.
Do java -version again et voila, 1.7!