When you change the EPG type from IceTV to Humax you will no longer receive the benefits of using the IceTV Guide and of the IceTV Interactive service for remote scheduling and series recording.
One thing that will happen is that all series recordings and timers that are on the Humax will be cancelled both on the Humax and on the IceTV website, so if you go back to using the IceTV Guide you will need to re-create all of your series recordings.