In such cases, you might have to restart it all over again. As a result, you might experience slow download speeds, and sometimes it might even get interrupted during the download. During the first few days of public release, the Apple server gets hit with a huge number of download requests. The initial experience with OS X upgrades is almost the same every year. Check out how to get macOS Sierra from outside the Mac App Store. That was after the MAS download got interrupted 6 times in a row, and I had wasted more than 15 GB of data on a single download of 6.08 GB. It was only a few hours back that I was successfully able to upgrade my Mac. However, my experience with the upgrade process was not a pleasant one.
The OS upgrade is highly recommended as it vastly improves the performance of Macs (that are capable of running Yosemite).
I was very excited about the release of OS X El Capitan earlier this week.
