A House-Senate conference committee yesterday amended District home rule legislation to deal with new fears that the measure as previously written might have overturned as many as 500 D.C. rape and sexual assault convictions.

The home rule provisions, which were tacked onto Congress' catchall continuing spending resolution, are needed to clear up confusion over the city's authority to govern itself and to prevent the city from being plunged into financial chaos.

Congress has been working on this corrective legislation for more than a year but only recently was able to end a stalemate over it in the Senate that resulted from Reagan administration opposition to the measure.

The conference finished its work on the spending resolution yesterday, and the House approved it late last night. The Senate is expected to approve it today and send it to the president, who is expected to sign it.

Sen. Arlen Specter (R-Pa.), who offered the amendment yesterday in conference, said the problem with the rape convictions was brought to his attention by U.S. Attorney Joseph E. diGenova only last week after the White House had reversed its opposition.

Specter said he was relying on diGenova's judgment that the change would improve prosecutors' positions in upholding rape cases currently being challenged in court on the basis of a 1983 Supreme Court ruling that brought the District's Home Rule authority into question.

In correcting problems that arose from that ruling, Specter said, it appeared that the home rule legislation as originally written might be interpreted as validating a sexual assault reform measure that the D.C. City Council had approved but which Congress overturned in 1981.

After that law was vetoed, prosecutors continued to use old District rape and sexual assault laws to prosecute cases. If the sexual assault bill were validated now, a court could say that rapists convicted since 1981 were prosecuted under the wrong law. Between 300 and 500 convictions are at stake, Specter said.

Pauline Schneider, D.C. director of intergovernmental relations, said city officials were not notified of the amendment until after the conference had adopted it.

"I think in principle we wouldn't have a problem with it . . . " she said. But, she added, the city's belief was that it was not needed.

The problem stems from a 1983 Supreme Court ruling invalidating legislative vetoes and specifying that legislative actions require the concurrence of both houses of Congress and the president.

Since the home rule act includes a one-house legislative veto for District-passed criminal legislation and a two-house veto for other legislation, the ruling brought the District's self-government authority into question.

Without self-government authority, the city could not issue bonds to borrow in the private markets as it had planned to do.

In addition, a number of persons convicted under D.C. criminal laws have challenged their convictions in court, saying the the laws under which they were prosecuted under were invalid.

Under the corrective measure, laws passed by the District could be overturned only with a majority vote of both houses of Congress and the president's concurrence.