It's theoretically possible to do that, but not with the Lexia software, you would need some special 3rd party software that ECU unlockers use that is capable of downloading the firmware and uploading arbitrary versions.
Firmware updates are a one way process only on a Lexia - it will only allow you to update the firmware if a newer version exists but if there isn't a newer version or you're already on the newest version you cannot force it to re-flash the same version, nor can you force it to downgrade to an older version.
I found this out the hard way when I updated the firmware in my Xantia gearbox and realised that one of the new "features" (listed cryptically in the update notes) is that it purposely holds first gear to 3200 rpm for the first two minutes of driving after a cold start to help heat up the catalytic converter quicker - something that I find extremely obnoxious when leaving for work as I end up driving the first quarter of a mile in 1st gear as I can't quite get fast enough for it to change to second while slaloming my way through the narrow parked car infested streets... I can override it with manual gear changes in snow mode but it's a pain.
If I could downgrade the firmware to the previous version I would, but I can't....
(at least not with a Lexia)
So anyone considering a firmware update via a Lexia beware - not only is there potential for it to fail completely and brick the ECU, even a successful update may make behavioural changes that you don't like and can't undo!
