One should check as frequently as time (and the value of your time) permits.

Fees are documented on the web site: http://www.delta.com/skymiles/use_mi...fees/index.jsp

Someone may correct me, but I believe any change other than date/time will trigger a fee for non-Medallion travellers. So a change to a more convenient routing (like xxx-CDG rather than xxx-ATL-CDG) will demand a fee.

As for when seats open up - there's no reliable answer to that question.

You were wise to take what you could get and plan a strategy for routing/date improvement.