If you have a big enough pipe to upload the entire ZD firmware image, it should work.
If the pipes a little small, it might just take a little longer.
However... it is often helpful if someone is actually able to view the ZD status LED,
so you know what you see on your upgrade screen, is the true current state. Flashing
red as firmware image is loading, ZD reboots, until solid Green when ZD is back up.
Some ZD1000/ZD1100 upgrades I've done, didn't always take me back to the login
screen when the ZD upgrade was completed. (so I also often run a continuous ping)
Second however... didn't you say that you were doing a DOWNGRADE(?), which if you
hadn't saved a backup from the version you are downgrading to, will result in the ZD
losing it's configuration, back to Factory Default state.
You will certainly need someone local to login and reconfigure the ZD if you lose the
config with a downgrade.