Data Domain Encryption

Hi we have recently enabled encryption of our DD2500s and I'm a bit confused as to exactly how encryption works when enabled but encrypt existing data has not been selected.

I understand at this point it is only encrypting new data, however I assume this means that once all the existing data has expired and been cleaned then we would be in a position where existing data has now been encrypted, does this mean the encryption progess section which is currently showing zero will have changed?

Also is there any way to show current encryption progress if you are only encrypting new data?