Under pressure from powerful committee chairman, Sen. Adlai E. Stevenson (D-Ill.) has agreed to several weakening changes in his proposal to chop the number of Senate committees from 31 to 15.

Stevenson made the concessions before the Rules Committee met yesterday to begin voting on items in the reorganization plan. It is designed to reduce overlap, reduce the number of committee and subcommittee assignments for each senator and end jurisdictional conflicts.

At meetings earlier in the week with Majority Leader Robert C. Byrd (W.Va.), Rules Chairman Howard W. Cannon (Nev.) and the Democratic chairman of other committees (no Republicans attended).Stevenson agreed to preserve three committees that his original plan would have absorbed into other units.

They are the Joint Economic Committee. Ethics committee and Joint Committee on Internal Revenue Taxation. If endorsed by the Rules Committee, this would mean at least 18 committees instead of 15.

At the same time, Stevenson agreed to increase the number of subcommittees on which a senator can serve.

His original plan allows a senator to serve on no more than two major comcommittees and one minor and limited him to two subcommittees on each major committee and one of the minor.

Stevenson didn't agree to change the limit on the number of committee assignments but he did agree to more subcommittee assignments. A senator would be allowed three subcommittees on each of his two major committees and two on his minor one - eight subcommittees altogether instead of five.

Moreover, it was agreed that an Appropriations committee member could belong to as many subcommitees as needed - a bow to powerful Chairman John McClellan (D-Ark.) who argued that his committee simply couldn't function under the original limits proposed by Stevenson.

The original assignment limits had been intended to reduce the number of subcommittees from 174 to about 100. The changes mean the reduction will be smaller and the wiping out of overlap correspondingly less.

Because of objections from lobbies or key chairman to shifts of functions from one committee to another, Stevenson agreed to make these other changes, all subject to ratification by the Rules Committee (perhaps on Monday) and by the entire Senate.

Veterans' Affairs to be absorbed by Armed Services instead of the proposed Human Resources (now Labor and Public Welfare) Committee. Veterans' groups seeking to preserve their committee may yet succeed either in the Rules Committee or on the Senate floor: if not, they have said they'd prefer being absorbed by Armed Services rather than a committee associated with welfare and the poor.

Small Business to become part of the Finance Committee, instead of the Agriculture Committee: this will allow it's present chairman, Gaylord Nelson (Wis.) to remain control because he is a member of Finance.

Post Office-Civil Service and District of Columbia committees to be absorbed by proposed Governmental Affairs (Government Operations) Committee, as Stevenson proposed earlier but giving the ranking Democrat and Republican on the two abolished committees the option of joining Governmental Affairs: the Democrats would head its subcommittees on Post Office and the District.

Irrigation and land use planning (now under the Interior Committee) to remain in Energy (Interior) instead of going to Agriculture; economic development (now in Public Works to remain in Environment (Public Works) instead of going to Agriculture; Bureau of Labor Statistics and coal mine safety jurisdiction (now in Labor) to remain in HUmna Resources (Labor) instead of being spun off to other units as Stevenson proposed.

Stevenson insisted that the changes don't "do violence to any of the major purposes - to equalize, rationalize and democratize the Senate." But others said privately that the concessions represented an important, if not mortal, chipping away of his design.