Fairfax County, which has been holding up ratification of the regional subway construction agreement, yesterday approved a new version that would require important concessions from its mass-transit partners.

The Fairfax Board of Supervisors, meeting in special session, rewrote key portions of the agreement in an effort to bring Metrorail service to the county as early as mid-1984. The original agreement, which everyone else has signed, could delay the start of service until 1985.

The Fairfax changes would require Metro to give the county a written guarantee that the West Falls Church Station will be ready in mid-1984 and that Vienna -- the last station of the so-called Orange Line -- will be ready within six months of that date. If the guarantee isn't given within a year, then Metro would have to refund to Fairfax almost $9.3 million in local share construction funds.

The changes also mean that Falls Church would have to accept the possibility of the West Falls Church Station being the end of the line for at least six months.

The city has already extracted an agreement from the Metro board that the station won't be opened until the rest of the line is ready. City officials say that making the station a temporary terminal would create a huge traffic problem and potential safety hazard for nearby George Mason High School.

Supervisor James M. Scott (D-Providence), who led county efforts to get a better deal on new subway construction, said the county's position represented "a harder line . . . This is not a bargaining position. This is what the county expects."

The changes now will have to be sold to suburban Maryland and the District as well as the rest of Northern Virginia, especially Falls Church.

While this could take time, it apparently is no longer crucial that the subway construction agreement be sealed by the end of the month to guarantee that $275 million in federal fund will flow to Metro.

Metro general manager Richard S. Page said yesterday that construction during fiscal 1979 -- the only period for which federal money is guaranteed -- will be able to go forward even if the Metro partners continue to disagree about the details of future construction.

"I don't think the fiscal 1979 program is in jeopardy," Page said. The Fairfax revisions, he went on, are "simply a little bit more specific statement than the Metro board agreed to last week."

Fairfax's concern for quicker action has been heightened because most other major Metro contributors already have some rail service.

The county's Metro representative, Supervisor Joseph Alexander (D-Lee) -- who helped forge the original construction agreement -- resigned his board seat earlier this week, complaining his Fairfax colleagues had lost confidence in him.

But heartened by a vote of confidence, Alexander reclaimed his seat and yesterday joined in the unanimous vote to take a tougher line.