iCloud backup restore from iOS 8.4 to iOS 9.3 possible?
There shouldn't be a problem. I believe I've done an iCloud transfer from iOS 5.1.1 to iOS 7.0.4, which is a much larger gap.

If anything, going from iOS 9.3.2 to iOS 8.4.1 might be a problem, but you aren't doing that, so you'll be fine.

iCloud is built with moving to a new device in mind. It wouldn't make much sense for Apple to make it impossible to restore a backup from a previous iOS version.