What does "slide to upgrade" do?
I just bought an iPhone SE. It came pre-installed with iOS 9.3.2. I set it up from an iPhone 5s iOS 8.1 iTunes backup. When the restore was complete, I was greeted with the "slide to upgrade" message. I slid it, and a loading bar with the Apple logo was displayed. When it was finished loading, a message appeared saying "upgrade complete." When the setup was finished, I went into Settings; it was still on iOS 9.3.2. It is now asking me to update to iOS 9.3.3.

Thus, "slide to upgrade" does not update your iOS firmware to the latest version, it likely just prepares the old backup data for use with with iOS 9.