Arrival in Paris

You'd need a miracle to catch that train. Change to sometime past 2:00 p.m., 14:00 on the train schedule, which you could make if your plane isn't late.

The fastest is the RER.

Unless your flight arrives about 2 hours early, you cannot make the 12:10 train no matter what you do. Change your train arrangements.

I always take the Roissybus. It picks you up right outside the terminal, and drops you off a few short blocks from Gare Saint Lazare. I find it the easiest, and fast compared to trying to get to the RER, then transfer to the Metro. Particularly at that time of day. Still that is cutting it quite close. There are just too many variables. You flight may be early, or it might be late. Customs/immigration may be short or long also.

I agree that I would change the train ticket

If you only have carry on luggage, and Jupiter aligns with Mars, you might make it. RER goes every 7 1/2 minutes, but you have to make a change, either the RER E at Nord, or the 14 at Les Halles. So figure an hour. Taxi is 50 EUR, also an hour if traffic is not backed up.

Sadly, all the options you list have drawbacks. Assuming you get through immigration rapidly ( where are you sitting on the plane?) unless you are very familiar with CDG, it's going to take some time to orient yourself to where you need to be for whatever option you choose. The taxi requires waiting in the taxi queue for at least a few minutes and then hoping there's no accident or backup, the others require a few minutes for buying tickets and making sure you're going in the right direction. RER/metro requires a change, and finding the change, Roissey bus is going to require getting your luggage off and then a few blocks...and then you've got to find the platform and maybe validate your ticket. And, what will it cost if you try anyway and don't make it?

I'm in the group advising a change.

Thank you all for taking the time to reply. For what it's worth, we will just have carry-on luggage, so baggage claim won't be an issue. Obviously, we have no control over whether or not the flight is on time or how long the line at immigration is. The train ticket is not changeable, but I didn't pay so much for it that I'm particularly concerned about it. If we don't make that one, the next trains are, I think, at 1:34 and 3:03. We'll get there one way or another.

Yes, you won't make it. It will be the 1:34 for you!

Where are you arriving from? Maybe I missed that but if you do not have to go through passport control and given you have only carry-on luggage, you have a much better, maybe even a good, chance of making the train. If you are ready to go by 10:40, my guess is you will make the train assuming no major traffic issue if you take a taxi.

The cost of buying a "walk up" tickets when you get there will probably be a great deal more than the ones you have already purchased that are not refundable. Since your original cost was low, and you can absorb the loss if you miss the train, consider another option. Buy another set of advanced purchase cheap tickets for a later train. Obviously you'll lose the money for one set of tickets, but that might be far less than the cost of "walk up" tickets when you get there. Run the numbers and see what you think. Don't know if there is any sort of penalty or "trouble" if you do not use a set of the tickets, but it would not hurt to check that out.

Of course there is no penalty for not using a train ticket. I would get another set of cheap PREM tickets if they are still available or just hope for the best if they aren't and plan to buy the walk ups at 3 times the price. If you have to go through immigration it is unlikely you will make this, but maybe the plane will be half an hour early and you will luck out at immigration.