Migrate old admin's posts to his new user
Node data is stored in node table also.. It's a bad practice to change node content directly using sql queries...

You can use Views Bulk Operations (VBO) module to change node authors in bulk...


  This module augments Views by allowing bulk operations to be executed
  on the displayed rows. It does so by showing a checkbox in front of
  each node, and adding a select box containing operations that can be
  applied. Drupal Core or Rules actions can be used.