Hello, I have booked a tour at Versailles and the tour company will provide us with a ticket from Versailles to Paris downtown. I am actually hoping to straight to CDG airport from Versailles.With the ticket we will be given, is there a way to upgrade (put more money on the ticket) so that we can go all the way to the airport on RER (transfer at St Michel Notre Dame) or do we have to get off then buy another ticket at St Michel Notre Dame RER station?I was told ticket from Paris downtown to Versailles is 4 Euros and Paris downtown to CDG is about 10 Euros, so maybe I thought I can just upgrade the ticket by paying 6 Euros?Thank you very much for your wonderful website :)
There's no ticket upgrade you can buy, but you can purchase another ticket while in Versailles that will be good to use when you arrive in Paris from Versailles, for your onward journey to CDG. (There are no direct trains from Versailles to CDG Airport, you must transfer, usually at Gare du Nord, but St Michel Notre Dame works as well.)
Getting help buying this ticket from a ticket window at Versailles train station would work as long as the ticket seller understands your situation. (i.e. you need a ticket from "Paris Zone 1" to CDG, already have Versailles to Paris ticket.) You'll still have to validate any new 2nd ticket you use while at St. Michel-Notre Dame station, else the fare gates at CDG won't let you out of the station into CDG airport.
I can't remember what the transfer is like at St Michel-Notre Dame (you'll be changing from RER C to RER B lines), but just make sure you use the 2nd ticket when entering any gates while inside St Michel-Notre Dame station. If this new 2nd ticket is accepted at this gate, you'll be able to exit the CDG Airport train station after arriving at the airport.
Good luck and thanks for the kind words about the site.