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I know that if i upgrade to firmware version 1.3.3 i can't downgrade to a lower version.
But I'd like to know if possible to downgrade from 1.3.4SR1 to firmware 1.3.3 because i'd like to test version 1.3.4SR1.
You should be able to downgrade from 1.3.4SR1 to 1.3.3. The restriction of 1.3.3 down is due to hardware changes in the phone that previous code does not support