As an Android developer I can tell you that switching versions on a device is not provided by root access. Root gives you access to all of the system while the phone is running. Getting a different version running requires an unlocked bootloader. For example, on my Nexus devices I first do a "fastboot oem unlock" to unlock the bootloader, then I can use fastboot to send any version of the OS I want to the device. It gets tricky on more locked down devices, but the same basic idea still applies. You need to get past the bootloader to get a different OS running.