I am not 100% sure as I have not updated and had to run through the process, but I believe you would have already had to have a 6.2 emunand for it to work with the SX 2.5.3 update. When you create emunand it will create it as 7.0 which is not supported yet. So I am not sure how you would boot to it and downgrade. I may be wrong though honestly.