var remark = require('remark');
var modifyChildren = require('unist-util-modify-children');

var doc = remark().use(plugin).process('This _and_ that');

console.log(String(doc));

function plugin() {
  return transformer;
  function transformer(tree) {
    modifyChildren(modifier)(tree.children[0]);
  }
}

function modifier(node, index, parent) {
  if (node.type === 'emphasis') {
    parent.children.splice(index, 1, {type: 'strong', children: node.children});
    return index + 1;
  }
}

This **and** that