Is it possible to downgrade iOS 8.3? Currently no! Recently it was impossible to downgrade iOS 8.3 to iOS 8.1.3 but from now iOS 8.2 is inaccessible for 8.3 users as well.
On Monday, Apple stopped signing iOS 8.2. As there’s always a short period after an iOS software release whereby the previous firmware version can be downloaded and installed. The older iOS 8.2 software is completely frozen out, and iOS 8.3 is currently the most recent public build for iOS devices users.
Often, it is sad to see the signing of an older firmware, as for example, said version may be jailbreakable. However, the closure of this particular signing window has no measurable impact on jailbreakers, it’s still worth nothing any time Apple closes a firmware signing window for users who don't have jailbroken device.
If, for some reason, you didn’t want to upgrade from an earlier iOS version to iOS 8.3, you could, up until Monday, upgrade to iOS 8.2. Or, if for some unknown reason, you wanted to downgrade iOS 8.3 to iOS 8.2, you could do so up until Monday as well. There is no advantage of downgrading to iOS 8.2 either since there is no jailbreak available for it. That is, of course, no longer the case.
Apple company patched the TaiG jailbreak in iOS 8.2 and has continued to do so in iOS 8.3 firmware.
Cupertino company started seeding betas for iOS 8.4 last month and has released two so far. iOS hacker, I0n1c, previewed a working iOS 8.4 jailbreak late last month and demonstrated a video of it, but that’s not going to be public launch. After several jailbreaks i0n1c’s efforts and the progress surrounding TinyUmbrella, give us hope that there’s more to come from the jailbreak community this side of iOS 9.
So iOS 8.2 is no longer being signed. It is not possible to downgrade iOS 8.3 to iOS 8.2. If your device currently runs iOS 8.1.2, it is best not to update to any new version since the last jailbreakable version currently is iOS 8.1.2.