Is it possible to downgrade M580 CPUs "new hardware" with old firmware (<V2.90)?

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In Jully 2019, we have published a new firmware SV2.90 for M580 PLC range. This new Firmware is required to manage a new embedded Switch (new hardware).
If you try to downgrade a M580 CPU (new hardware) with a firmware <V2.90, Unity loader will reject the downgrade.
New M580 CPU HW can’t be downgrade with old FW(<2.90) whatever the Unityloader version.