Yes, you can roll back to 0.6.3 without physical access to the device.
First flash user firmware that targets 0.6.3 or earlier, so you won’t immediately be upgraded again.
Download system part 1 and 2 from the release site:
Flash them in the reverse order using the CLI:
particle flash DEVICE_NAME_OR_ID system-part2-0.6.3-photon.bin
particle flash DEVICE_NAME_OR_ID system-part1-0.6.3-photon.bin
Note that you must flash part 2, then part 1 when downgrading.
Also, if you want to downgrade farther, to say 0.6.2, you must downgrade to 0.6.3 first, then 0.6.2 in two steps.