Hi Yijie .. I'm not a microcode guru .. however AFAIK you can't choose to downgrade your code. If you are unhappy with a new code, you have to open a SR and tell EMC why you are unhappy. EMC will usually fix the broken code (in case it's broken) or help you workaround what's giving you PITA.

Usually new codes fix old bugs .. thus usually new codes offers more features and better performances of existing features. I see no reason to downgrade. And even if you can UPGRADE online, I guess DOWNGRADING can be only OFFLINE. But I'd love to be wrong ...