In one of the most eye-catching of President Barack Obama’s proposals Monday to change the tax code, he suggested replacing the widely disliked alternative-minimum tax with a 30% minimum tax on people earning at least $1 million, based on the so-called Buffett Rule. The impact of the AMT change could be dramatic. A household of four with income of $500,000 could see its tax bill drop to $124,306 from $142,050 under current tax policies, according to an estimate by the nonpartisan Tax Foundation. That assumes full AMT repeal. Mr. Obama’s budget without the AMT repeal would raise their taxes to $147,800.