Downgrade from 10.2.0.4 to 10.2.0.2

I did a clean upgrade from 10.2.0.2 to 10.2.0.4

I set the compatible parameter to 10.2.0.4 and opened db and tested upgrade.

To downgrade I get the following:

Error:   ORA-39707  (ORA-39707)
Text:   compatibile parameter %s too high for downgrade to %s
---------------------------------------------------------------------------
Cause: A downgrade was attempted, but the compatible initialization
parameter value was greater than the downgrade release version.
Action: Once the compatible value has been raised, downgrade to earlier
releases is not supported.

How are you supposed to test an upgrade if you don't bump up the compatibility parameter?