It’s important to note that this is not an automatic installer—this process will only download the update in the background, and then alert you that it is available to install. You can choose to install it when its convenient. You can also choose to ignore the update.
The update will only be download on systems that support the new operating system. It will not download on systems that are low in space and if your system starts to run out of space the download will be deleted.
You can also manually delete the download if you don't want to upgrade and manually update at any time.
Apple plans to roll out auto download over the next week.