There are several TGV trains a day that run from the train station under CDG Terminal 2 to Avignon, so time to station really depends on which terminal you're landing at. Personally, I'd leave an hour to get thru customs and pick up baggage, a half hour to get to the train station, and another hour for padding....in case, for example, your flight arrival is delayed. Some of the trains will take you all the way to Avignon center, others will drop you at the Avignon TGV station, which is a short bus ride in. Whether you can change your ticket depends on the ticket you buy...the cheaper the ticket, the more restrictions on it. You can check for all the details, and purchase, at the French national railway website, http://www.voyages-sncf.com and click on the Union Jack in the lower left for English. Enter "Roissy" as your departure station.