(Rewrites first paragraph, updates prices, adds quote, changes byline, changes dateline from previous BEIJING/PARIS)

CHICAGO, June 29 (Reuters) - Chicago spot corn and soybean futures extended gains on Wednesday, as traders adjusted their positions ahead of a key government crop report, traders said.

But the most-active and deferred corn contracts eased downward, as traders expect the U.S. Department of Agriculture to raise its estimate of domestic corn plantings in its acreage and stocks report on Thursday.

Analysts also expect the USDA to show a cut to the estimated soybean and spring wheat areas.

Adding to the day's volatility was investors rolling off the July contracts ahead of first notice, or the first date users get notified on being assigned a delivery of their contracted grain, said Karl Setzer, commodity risk analyst for AgriVisor.

"Tomorrow is a heck of a day, with the USDA report and first notice - and also month end and quarterly end," Setzer said. "That's why we see July corn up, and the rest of the contracts trading lower."

The Chicago Board of Trade (CBOT) most-active corn contract was down 4-1/4 cents at $6.55 a bushel at 10:03 a.m. CDT (1503 GMT). The July corn contract was up 10-1/2 cents at $7.70 a bushel.

The most-active soybean contract was up 9-3/4 cents at $14.72-1/4 a bushel.

Meanwhile, Chicago wheat futures eased after Sovecon agriculture consultancy raised its forecast for Russia's July-June wheat exports by 300,000 tonnes to a new record high of 42.6 million tonnes on Wednesday.

But signs of improving demand helped put a floor under wheat prices, traders said.

Jordan's state grains buyer purchased about 60,000 tonnes of hard milling wheat to be sourced from optional origins in a tender that closed on Tuesday. And Bangladesh, Pakistan and Egypt launched tenders to purchase wheat.

The most-active wheat contract on the Chicago Board of Trade (CBOT) was down 3-1/4 cents at $9.32-3/4 a bushel. (Additional reporting by Hallie Gu and Dominique Patton in Beijing and Sybille de La Hamaide in Paris; Editing by Mark Porter and Lisa Shumaker)