When one did an upgrade ever since Windows 10 began Microsoft offered the opportunity to recover their system by rolling back in upgrade. They now have shortened it to just 10 days instead of the 30 that they had done for previous versions.
They must figure that if you like Windows 10 you're going to really like the new anniversary update. You probably already know what Windows 10 is all about so you shouldn't need to have the longer period to check things out. If you are unhappy with the new version you should probably be able to figure that out within the 10 day timeframe.
According to winsupersite.com, a Microsoft spokesperson it was a data-driven decision to change rollback timeframe from 30 to just 10 days:
According to Microsoft they were seeing that people were rolling back in less than a week if they were going to do so and so in their mind, they were helping the end-user to recover somewhere between three and 5 GB of storage space on the device that was being used by the rollback recovery. This could be very significant for low storage space devices so probably is a good thing that this has been shortened to help you out. I'm sure a lot of you will be able to come up with a case for why this would not be a good thing. ."Based on our user research, we noticed most users who choose to go back to a previous version of Windows do it within the first several days. As such, we changed the setting to 10 days to free storage space used by previous copies."