Train rules for tgv train travel from Paris Airport to Tours?

I am planning on travelling on a tgv train from the CDG Paris airport to Tours France. If I purchase my ticket in advanced, and I miss the train because my flight was delayed will I be able to use the train ticket on a train later in the day?

it depends of the ticket type, please refer on sncf website...

Most train tickets allow you to change your reservation until the day of departure (but not AFTER the train has left) but the cheapest fares are not refundable nor can they be changed. Your initial ticket won't be valid though, you'll have to exchange it for a new one at the ticket booth. There will most likely be a reservation fee. Note that it won't be possible if your train has already left so if I were you I would book on a later train and then try to change your ticket for an earlier one once at the station.
http://www.sncf.co.uk/

Another thing you can do if you don't have too much luggage is take the subway from CDG airport to Paris and take a regular train (not TGV) from Gare d'Austerlitz train station. It takes longer (2hours 37) but there are trains every hour and you can buy your ticket when you arrive at the station. On a busy day you are more likely to get a seat than on a TGV.