index.js

const R = require('ramda');

const userRegAttrs = [
  { reg_id: 1, attr_name: 'foo', attr_value: 1 },
  { reg_id: 1, attr_name: 'bar', attr_value: 3 },
];

const user_id = 1;

const res = R.pipe(
  R.map(R.omit(['reg_id'])),
  R.map(R.mergeRight({ user_id })),
)(userRegAttrs);

console.log(res);

// [
//   { user_id: 1, attr_name: 'foo', attr_value: 1 },
//   { user_id: 1, attr_name: 'bar', attr_value: 3 }
// ]