So here is the problem, if someone on this forum says you can miss those train journeys, are you going to trust them? You risk having to pay a $300 change fee or even worse having your return flight cancelled and having to pay much more. It seems you are very cost conscious, nothing wrong with that at all, but be careful what risks you take. If there is a big difference in fare by all means consider taking that fare. If you have reasonable connecting time in Paris the rail trip would be quite comfortable and there is nothing wrong with adding a day in Brussels to your itinerary. It would take a big saving for me for that to be worthwhile, just because the last thing I want to do when I arrive in Europe is to hang around a train station for a couple of hours, if it can be avoided.