Is it possible to upgrade, without rooting, to a version not specifically released for the device?
Most phones can't upgrade (officially) without a signed firmware package designed to upgrade from the existing version of Android that's already on the phone.  In those cases, you can't upgrade without rooting and/or unlocking your bootloader and/or flashing a custom recovery, which all void your warranty.

Some phones may not be as limited; for example, the original software on the original Galaxy S devices allowed unsigned updates to be flashed without needing to unlock the device.  Theoretically you could try to flash a firmware package from a different region; it's hard to say how that would affect your warranty (and besides, that's really off-topic here :P).  You could also of course flash a custom ROM with a newer version of Android, which again will void your warranty just like rooting.