Oracle Database Discussions

I am testing setting up transparent data encryption and I have been setting up some test tables to see how long it will take to encrypt some big tables. My question is...does setting up the column as encrypted actually encryot the existing data in the table or is it just for new data that is added once you set the column as encrypted?
I have been testing on some tables that already have data in them and it returns immediately after issueing the later table command and it does not seem to matter how many rows are in the table?
I wa sunder the impression that there were be a conversion of the existing data? Is this not true and does TDE only affect newly added rows?
Thanks.