It's possible to keep the "Lei Shi's" Haste buff, even after you switch modes, aka 10M Heroic, to 10M normal.
What you have to do:
Switch Heroic, execute phase 1 properly and reach phase 2.
When you get zoned into the shadowy/Watery Realm, you should be applied with Lei Shi's hope for every single attempt, up to an hour after the last person left the raid-group.
However, if you reach phase 2 on heroic, you can switch to normal while still standing inside the raid and keep the buff.
This will make it easier for those who're able to reach phase 2 on heroic, but are going for the "no fear" achievement, or simply WoL Ranks on Normal version.