There will be an important change in the discounted fares elderly and handicapped citizens pay when riding Metrobus and the Metro subway beginning next Friday.

Today, on the bus and train, the elderly pay half-fare, but get their discount only during the off-peak hours. During rush hours they either have to pay full fare or they do not ride.

But starting Friday, special rates will be provided elderly and handicapped citizens at all hours. As is true for everyone else, however, figuring the fare is not that simple.

For the subway, elderly and handicapped will pay half of the rush-hour fare at all times, computed downward to the nearest nickel. For example, the most expensive rush-hour subway ride will cost most people 65 cents, but those who qualify for elderly and handicapped fares will pay only 30 cents at all hours.

There's a catch. Metro's computerized fare-collecting system on its subways requires the use of a special ticket, called a Farecard, both to enter and leave the subway station. To take advantage of the special fares, elderly and handicapped will have to buy specially encoded Farecards. The special Farecards are not for sale in subway stations. A list of sales outlets for special Farecards follows this article.

For buses, at all hours, elderly and handicapped people will pay half of the regular off-peak fare.

That means that a bus ride within the District of Columbia, or within Virginia or within Maryland will cost 20 cents at all times; bus rides between the District and Virginia will cost 35 cents at all times and bus rides between the District and Maryland will cost 30 cents at all times.

Senior citizens, defined by Metro as anyone over 65 years of age, can qualify for the special rates by picking up an ID card at any area public library. Senior citizens must bring some proof of age with them to qualify, and will get a card identifying them as senior citizens.

With that card they can (1) board a bus and pay the discounted fare and (2) purchase discounted Farecards for the subway system.

Handicapped people can obtain an application to qualify for discounted fares at one of the locations listed at the end of this article or by calling Metro's Handicapped Service Unit at 637-1245 or 637-1246. An application will be mailed.

The application must be completed by a licensed physician, and is subject to review by a Metro staff doctor. When the application is completed, the handicapped person can call for an appointment at Metro or come to Metro Headquarters (600 5th St. NW) on a first-come, first-served basis to be photographed for a laminated identification card.

Possession of that card entitles the handicapped person to board a bus at discounted fares and to buy the special reduced-rate subway Farecard.

Special-rate Farecards for the subway can be purchased for $4 each, and will entitle the rider to about the same amount of Metro trips as an $8 card would to another person.

Properly qualified handicapped and senior citizens can buy the discounted Farecards at these locations:

Metro Headquarters, 600 5th St. NW; Metro sales office, 1422 New York Ave. NW; Washington Bus Terminal office, on 10th Street between Constitution and Pennsylvania Avenues NW; the Bladensburg Sales Office, at the rear of 2250 26th St. NE; the Pentagon sales office, B-5 Stairway, Pentagon Concourse; Northern Bus Division, 4615 14th St. NW; Western Bus division, 5230 Wisconsin Ave. NW; Southeastern Bus Division, 17 M St. SE; Royal Street Bus Division, 600 N. Royal St., Alexandria; Four-Mile Bus Division, 3501 S. Glebe Road, Arlington; Arlington Bus Division, 707 N. Randolph St., Arlington, and the Maryland Bus Division, 4421 Southern Avenue, Coral Hills.