|What do you mean by "is no good anymore" ?|
Both of the above are operating systems on their own right. You cannot just throw a switch and revert from one to the other. If you need to change then you must have the installation cd and license for the operating system to be used.
To protect your personal data you need to save it to a different media than what it is currently stored on...CD or and external hard drive , flash drive etc. Then either restore the current operating system or install a different one.
Change Is Good