NEW YORK (AP) _ International Business Machines Corp. jumped its own schedule Thursday and shipped the first upgrade for its 3090 Model 400 mainframe, which it bills as the most powerful generally available, general-purpose computer in the world.

The early shipments should add to IBM's revenue this year, but the company is still likely to finish 1986 with profits below those of a year ago, said James Meyer, an analyst for Janney Montgomery Scott Inc. in Philadelphia.

IBM had originally said shipments of the big mainframe would begin in the April-June period of 1987, but had recently been indicating to analysts that shipments would begin around the beginning of October this year, Meyer said.

The first Model 400 will be placed at A.C. Nielsen Co. in Green Bay, Wis., as an upgrade to a smaller Model 200 already installed there, IBM spokesman Tom Belz said.

The upgrade kit was shipped from an IBM plant in Kingston, N.Y., on Thursday, he said.

Shipments of factory-built Model 400s, as opposed to kits for upgrades, should begin in the October-December quarter of this year, Belz said.

Belz said the computers were being shipped early because testing was completed ahead of schedule and customers were pressing for early delivery.

IBM announced the 3090 series of mainframes, nicknamed Sierra, in February 1985. The Model 400 has four interconnected processors and is twice as powerful as the dual-processor Model 200, IBM says.

Although IBM does not list the speed of its computers, independent analysts say the Model 400 can perform about 52 million instructions per second, depending on the task.

Supercomputers such as those made by Cray Research Inc. can run faster, but they are usually for scientific research or other number-crunching operations. The Model 400 runs standard programs.

A factory-built Model 400 sells for $7.9 million and a Model 200 for $4.1 million, and upgrading to the bigger model from the smaller one is $3.8 million, Belz said.

A.C. Nielsen will use its Model 400 to process data garnered from 1,700 supermarkets that use bar code scanners to read product labels, Nielsen spokesman Robert Bregenzer said. The computer compiles and reports information about how well different products are selling in various markets, and the Model 200 used for the job now has reached capacity, he said.