Interesting - so in the latest smart update Avaya released firmware for i2000 series phones (or at least for the i2007) with their own branding.
I don't believe that firmware for i2000 series phones was ever locked to disallow downgrades. Unless Avaya added this "feature" to the firmware, you should be able to downgrade the firmware installed in the phone just by forcing firmware download on a system with older firmware (without the latest patch). If you have another BCM without the latest patch applied, you can try to connect the phone to that BCM and use the "force firmware download" option from EM.
If the phone accepts the older firmware (and comes up with the Nortel branding), the next step would be to disallow firmware updates (or to remove the new Avaya firmware from the BCM50). I don't remember if BCM50 release 3 always updates firmware automatically or if there is a way to disable firmware updates.
First, try the downgrade the firmware in the phone by using another BCM. Considering options for the second step would make sense only if the downgrade is successful.