The Metro board took a giant step yesterday toward resolving one of it's oldest issues when it officially adopted two changes in the routing of the subway's Green Line from the District of Columbia border to Greenbelt in Prince George's County and thus cleared the way for final design work.

The changes, much sought by Prince George's residents in a series of highly charged public hearings last summer, involved slgiht alterations in where the subway will run plus relatively minor changes in the location of the both the West Hyattsville and Prince George's Plaza stations. As a result, more than 200 apartment units, a church parking lot and three residences will be saved, along with $10 million in construction costs.

The board's approval doesn't guarantee construction of the section. Still needed are:

A final environmental impact statement, which the Metro staff is new authorized to prepare.

The resolution of a continuing debate in College Park about how to provide street access across the Metro (and Chessie System) tracks at Calvert Road upon completion of the College Park station. Metro's position is that the solution is a country problem. Metro would either build a bridge and keep Calvert Road open, or credit the same amount of money to build another road, as many in College Park have urged.

Money from the federal government to build the line. Proposed Reagan cuts in Metro's construction program would push the line to Greenbelt into the 1990s if it is built at all.

A decision on how the Green Line should run from the Prince George's-D.C. boarder to the Fort Totten station in the District, the nearest possible connecting point to the rest of the Metro subway system. The cheapest route is for an elevated rail line through parkland; residents of that neighborhood have lobbied hard for a tunnel. The difference in cost is at least $100 million.

Despite those problems, however, the board's action yesterday is a major step that reflects literally years of work between Metro and Prince George's County officials, whose support for the line has grown over the years as they have watched Metro's positive impact on tired communites. Albert J. Roohr, chief of system planning for Metro, recalled yesterday that he attended his first meeting on a possible line to Greenbelt in 1963 when he worked for one of Metro's predecessor agencies.

The Green Line would start at Greenbelt, run southeast into the District, cross the Red Line at Fort Totten and dive through Northeast Washington into the inner city. It would run from 14th Street NW at Columbia Heights south to Y Street, east to 7th Street, south to M. Street SW, east to the Navy Yard, south to Anacostia and St. Elizabeths Hospital then south into Prince George's County generally along Wheeler Road to Rosecraft Raceway.

Two widely separated segments of the Green Line are now virtually approved ast the local level: the section from the D.C. line to Greenbelt an an inner city link between 14th and U street Nwy and Anacostia. The rest of the line is tied up in political indecision in the District and litigation in Prince George's County.

Some ammunition for trimming Metro was provided yesterday by the Metropolitan Washington Council of Governments, which is in the midst of a highly detailed restudy of Metro's ridership projections.

In preliminary numbers released at a Metro board committee meeting, COG projected that if the full 101-mile system is completed, Metro's average weekday ridership would be slightly over 1 million, 20 percent below a 1978 projection that forecast 1.3 million riders.

The lower forecast comes because of lower than expected population growth and household formation in the metropolitan area. However, the biggest drop from the 1978 projections is in so-called "nonwork" trips. Metro and COG planners are having a civilized but major dispute over that projection because the subway has already generated many more midday trips than originally expected and the Metro staff expects that kind of travel to grow, particularly after the Red Line opens to upper Northwest Washington in 1982.