I have many of my train days booked. There are a few that I don't have booked and wonder if that will be a problem? I should just be able to go to the station and buy a ticket?
One is from Ypres Belguim to Paris on July 5?
Ypres is now called Ieper. Your fastest route from Ieper to Paris is Ieper-Kortrijk and Kortrijk-Lille Flandres on IC trains (no reservations are possible) and Lille Flandres-Paris Nord on a TGV. See the timetables at www.bahn.de.
The only leg for which it would advantageous to book in advance would be Lille-Paris in order to get a discount fare. However, Lille-Paris TGVs run about once/hour so you should have no problem walking up and buying a ticket at the station.