How to upgrade dataguard primary and standby databases

When you have dataguard setup in 10g version, would like to upgrade both primary & standby to 11g 
The idea is that you install the 11g binaries on both primary and standby;
shutdown the primary and the standby;
mount the standby off the 11g home,
 start the MRP; upgrade the primary.
 So the standby gets upgraded implicitly, by applying the redo generated during the upgrade of the primary.