Some information for those of you who do your own maintenance on Apple computers--this may only be Ken. If for any reason you think you may want to reinstall the original operating system that was on your new computer, be certain to download the desired installer to your machine and most preferably to an external disk that has an operating system installed on it. Once Apple releases a new version of the OS the links to previous versions disappear. If you have an installer copy you have the ability to go back to that system as it will download what is needed from Apple without problems.
If you have downloaded the OS previously you MAY be able to download the installer again from the App Store. If you have not previously downloaded it you are simply out of luck. Only the latest iteration will be available from Apple.
Of course one can always resort to witch doctors and exorcists but I have doubts about their reliability.
Likewise if you are one of those many folks that only run the latest, greatest version of everything, you don't care.