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I'm still using Android 7 (Nougat) on the X Compact, thinking about upgrading to Android 8 (Oreo).
But I'd like to be able to revert back to 7 if I don't like it.
Can I do a complete system backup and then eventually also restore this backup?
Nope, and you can't revert back with Xperia Companion.
With that said, you can use Xperifirm to downgrade / revert back to a different android flavor and/or regional firmware.
That's a shame. Xperifirm only lets me download the latest firmware, so I'd have to get 7.1 from a 3rd party now.
Or is there a copy of the firmware still on my phone, perhaps, from the last time I did an OTA update?
Not directly from Sony, though. Sony only offers the latest firmware.