Usually when Apple releases a major update, they leave the signing intact for the previous version for a few days or several days or even a week or so, so you can downgrade.
For example, I upgraded from 7.1.3 on my iPad 2 to iOS 8.0, but then decided iOS 8.0 was too slow and buggy and two days later downgraded back to 7.1.3. However, fast forward a couple of weeks later, and that was no longer possible.
BTW, 8.1.x was OK, but 8.3 is quite slow on that thing.