You will not be charged any more than the change fee, unless you can not get a flight until the next day. Call the airline toll free numbers to see if your plane is delayed for any reason, especially if there is bad weather. They will usually call if they know it is going to be canceled, if it is far enough in advance. Weigh these facts when deciding on a type of ticket and the associated price. Some travel agents will not check charter flights unless they are specifically asked to.
Charter flights can, in some cases, offer a cheaper price than a flight with a change-over.
Some travel agents do not have access to all airlines or to discount airlines. If there are weather delays for the rout you have booked, consider traveling to a city near your destination and renting a car one-way. Sometimes they will not rent you a car one-way at the counter, but you can through there toll free reservations. You may be delayed longer than you think. The airlines do not add flights, to make up for ones that were canceled.