Arlington County lawyers have approved the county school board's plan to amend a controversial 1971 desegregation plan that requires only black children to be bused but have cautioned that the proposed changes will lead to racial imbalances at some schools.

In an opinion released yesterday, County Attorney Charles Flinn and Assistant County Attorney Cynthea L. Perry cited legal precedents that they said show the county is not bound to continue its court-approved plan indefinitely without change.

Flinn and Perry stated in their three-page letter that the proposed changes do not violate equal protection guarantees of the 14th Amendment but do "create a situation in which the board needs to give more attention to racial balance in some schools."

The board, which previously directed its staff to propose the changes, is expected to approve them tomorrow night. The Arlington plan--not formally monitored by the county or amended since its approval by a federal judge 12 years ago--came under attack recently from blacks who charged that their children were bearing the burden of desegregating the county elementary schools.

The proposed changes would allow about 59 black kindergarten through third-grade students who live in South Arlington to attend Barcroft, Abingdon and Randolph Schools in that part of the county next year. Currently those children spend as much as 40 minutes each morning in trips to elementary schools in North Arlington.

According to the statistics included in the lawyers' letter, the proposal would increase the percentage of black students at Barcroft to 26 percent, more than double the current ratio. It would also cut the number of black students at McKinley Elementary in North Arlington, reducing the percentage there from 11 to 4 percent.

"Although we do not think the reduction to 4 . . . percent black enrollment makes the school in question all white, it does appear that the school deserves special attention so that steps to prevent further racial imbalance can be taken where possible," the lawyers said.

School Board Chairman Evelyn Reid Syphax, who has acknowledged concerns that amending the plan could resegregate some Arlington schools, said yesterday the system's projections for significant enrollment increases later in the decade would help balance enrollments. "It'll eventually work itself out," she said.

The school board is also expected to conclude a year-long school consolidation process tomorrow by voting on proposed changes in secondary school attendance boundaries.

Superintendent Charles E. Nunley has recommended phasing in the changes, generally shifting students to Yorktown High in North Arlington, the county's smallest high school. Only 42 to 50 students would be initially affected in the fall of 1984, according to School Operation Director Henry Gardner.

Nunley revoked an earlier recommendation to transfer Wakefield High students north of Long Branch and Lubber Run Recreation Center to Washington-Lee High, saying that transferring those students, most of them white, would not improve ethnic and racial balance in the schools.

The school board had directed Nunley's staff to revise the boundaries in order to equalize enrollments at the three high schools and improve ethnic and racial balance. Nunley said in his report that the proposals would have little impact regarding the latter.

He included with his recommendations a request for a demographic consultant to work on improving the system's racial and ethnic balance.