我正在做一个Express项目，并使用Sequelize作为我的ORM，我定义了以下Sequelize模型：

Field.init({
  type: {
    type: DataTypes.STRING,    
    allowNull: false,
    unique: true,
  }
}, {
  sequelize,   
  underscored: true,
  tableName: "field",
});

假设现在我想限制type列的字符量，并创建如下所示的迁移（async up是这里的焦点）：

'use strict';
const { Transaction } = require('sequelize');

module.exports = {
  async up(queryInterface, Sequelize) {
    return queryInterface.sequelize.transaction({
      isolationLevel: Transaction.ISOLATION_LEVELS.SERIALIZABLE,
    }, async t => {
        await queryInterface.changeColumn('field', 'type', {
                type: Sequelize.STRING(400),
                allowNull: false,
            },
            { transaction: t },
        );
    });
  },

  async down(queryInterface, Sequelize) {
    return queryInterface.sequelize.transaction({
      isolationLevel: Transaction.ISOLATION_LEVELS.SERIALIZABLE,
    }, async t => {
        await queryInterface.changeColumn('field', 'type', {
                type: Sequelize.STRING,    
                allowNull: false,
            },
            { transaction: t },
        );
    });
  }
};

unique和allowNull约束是否会保留在列中？或者这些会被删除，因为在迁移中我只列出了列的新类型？