CHARLESTON, W.Va. (AP) _ Union Carbide Corp. today confirmed it had replaced the head of its agricultural division responsible for major poison gas leaks in Bhopal, India, and Institute, W.Va., in the last nine months.

Meanwhile, GAF Corp., another chemical company, said that it had increased its stake in Union Carbide to 9.9 percent and that it was seeking regulatory approval to acquire as much as 15 percent, according to Dow Jones News Service.

Robert Oldford, who was president of Union Carbide's Agricultural Products Division when the leaks occurred, was moved late Thursday to a ''special assignment,'' company spokesman Dick Henderson said.

Oldford, who headed the division since 1978, was succeeded by Eugene Boros, vice president of Union Carbide's Research Triangle Park laboratories in Raleigh, N.C., Henderson said.

Oldford was out of the office and unavailable for comment today, a secretary said.

More than 2,000 people died after methyl isocyanate leaked at the company's Bhopal plant last December and 135 were injured at Institute this month by a leak of two pesticide ingredients. Union Carbide officials last week blamed the Institute leak on a combination of poor management and equipment failure, saying a vat of toxic chemicals was allowed to boil unnoticed for 11 days before pressure valves blew out.

At least eight other leaks have been linked to Union Carbide plants in West Virginia's Kanawha Valley since Bhopal, half from the agricultural products plant in Institute. The rest were at specialty chemicals units in South Charleston.

Oldford's reassignment came a day after the company announced it would eliminate 4,000 white-collar jobs through early retirement and voluntary severance plans.

Union Carbide's overall retrenchment program will reduce the number of salaried employees by 15 percent, the company said.

The retrenchment program will reduce the number of salaried employees by 15 percent, Carbide said.

GAF said today in a filing with the Securities and Exchange Commission that it is making the required application under the Hart-Scott-Rodino Antitrust Act with the Federal Trade Commission and the Justice Department seeking approval to acquire more than 10 percent of Union Carbide's stock but not more than 15 percent, Dow Jones said.

GAF, which is based in Wayne, N.J., announced the day after the Aug. 11 Institute leak that it had purchased 5.6 percent of Union Carbide's outstanding stock. Union Carbide announced the reorganization Wednesday, one day after GAF said its Union Carbide holdings had increased to 7.1 percent.

In addition to announcing the planned layoffs, Union Carbide said it would use $990 million from anticipated earnings and company pension funds in an attempt to buy back 10 million shares its own stock.