You Can't Downgrade to iOS 8.3
You need to be especially careful now that the iOS 8.3 downgrade has been plugged up by Apple.
Earlier this week, Apple stopped signing off on the iOS 8.3 update. What this means is that you can no longer drop back down to iOS 8.3 from iOS 8.4. Once you install iOS 8.4 on your tablet, you're going to be stuck on iOS 8.4.
This is important to note if you're coming from an older update like iOS 7 that's performing nicely on your older iPad. It's also worth noting if you're already on iOS 8 having a grand old time. This is precisely why you'll need to dig for a bunch of feedback before making your move, especially if your iPad is tied to school or work.