In my case I downgraded DFL61_HARMATTAN_40.2012.21-3.454.6_PR_454 to DFL61_HARMATTAN_40.2012.21-3_PR_001, so yes, it should be quite possible.
great, then I am (almost) ready to brick my device hehe. FYI. I have no need to really downgrade. Everything is in perfect condition. This is purely interest in the subject, but also, perhaps, helpfull for those with fx Chinese FW