Congress probably will make substantial changes in President Carter's new tax package - enlarging the tax reductions and scrapping most of the "reforms" - according to initial reaction in the House Ways and Means Committee.

An informal sampling showed key panel members almost unanimous in predicting the President wouldn't get more than a handful of the $9 billion in tax "reforms" and other revenue-raising measures he has proposed.

At the same time, several members - both Democrats and Republicans - warned that Congress may increase the size of the actual tax reductions. And some expressed doubts over whether Carter would win approval of his $240 tax credit.

Major congressional revamping of the Carter tax package could prove a major setback for the administration. Carter has proposed $33.9 billion in actual tax cuts, offset by $9.4 billion in revenue-raising measures.

If Congress refused to approve the "reform" measures - as many Ways and Means members believe it will - the net cost of the tax cut would rise substantially over the $24.5 billion Carter has alloted, bloating the budget deficit.

The President said in proposing the tax package that if Congress failed to approve the revenue-raising portion, he would want the size of the tax cuts trimmed. However, yesterday it appeared unlikely he will get his way.

Significantly, the Ways and Means members were almost unanimous in predicting that Congress will reject the three key elements in the President's "reform" package - ending two big foreign tax breaks and cracking down on business lunches.

Typical of panel member's pessimism was the reaction of Rep. Abner J. Mikva (D-Ill.), one of the committee's most liberal members, who until this year has been a supporter of broad-scale "tax reform."

Mikva said yesterday he doubted whether the committee would approve much more in "reforms" than a small crackdown on business deductions for yachts and sporting-event tickets, and possibly the new taxable bond option for states and cities.

"There's simply no constituency for tax reform," Mikva said.

Rep. Barber B. Conable (R-N.Y.), the ranking minority member of the panel also predicted rough sledding for the Carter proposals. "There's a considerable jack of enthusiasm for moving forward on the tax package," Conable said.

The committee scheduled hearings on the Carter tax plan beginning Jan. 30, but Rep. Al Ullman (D-Ore.), chairman of the panel, indicated it wouldn't start drafting legislation until after the energy bill has been passed.

The Jan. 30 hearing will allow for presentation of the tax package formally by W. Michael Blumenthal, the Secretary of the Treasury. In February, the committee will begin hearing public witnesses.

Rep. Joe D. Waggonner (D-La.), a conservative member of the panel, expressed apprehension that Congress may end up enlarging the tax-cut portion of the package, while at the same time rejecting the revenue-raisers - bloating its cost.

Waggonner joined other panel members in complaining that "there's too much reform" in the proposal.