Bob,
The website for the French train system is www.sncf.com/en. Some Americans have problems getting their purchases processed on that site. There is an alternative site for purchasing tickets that generally has the same prices and it is easier to use: captaintrain.com. For a primer on the French train system you can look at http://www.seat61.com/France-trains.htm#.VrLRVVMrIY0. A TER train is a local train that makes frequent stops and does not take reservations as compared to a TGV (Train à Grande Vitesse) which is a high speed train that requires reservations and makes relatively infrequent stops. Prices on TER are constant but prices on TGV fluctuate or rise as the travel date approaches or more specifically, the cheaper seats sell out sooner. There are also Intercités trains. As it says on the SNCF site, these are "the classic French rail service, linking major towns and cities across the country, with 340 Intercité trains serving medium and long-distance routes." Some Intercités trains require reservations and some do not. Usually, trains on short-distance routes do not and ones on long-distance routes do. You would find out when you go to buy the ticket. You could theoretically have a trip in which more than one type of trains run on the routes -- for example you can take a TGV to Dijon from Paris and get there in about 1.75 hours or less or take a TER and get there in 3 hours.
"If European train schedules change in June, what happens to the arrangements I would have already made for August?" Have you already bought train tickets? Although generally speaking tickets go on sale 90 days before the travel date, there are often tickets on sale more than 90 days before travel date for some summer travel. If by arrangements, you mean travel plans, generally speaking, absent track work or something similar or seasonal trains, the schedules do not change that much from season to season.