How can I call hook_user_presave() only when an account is updated, not created?
On creation $account will have is_new set to true. And other minor differences, like it'll not have created property set yet.

object(stdClass)#92 (18) {
  ["uid"]=&gt;
  int(0)
  ["hostname"]=&gt;
  string(3) "::1"
  ["roles"]=&gt;
  array(3) {
    [2]=&gt;
    bool(true)
    [3]=&gt;
    int(0)
    [4]=&gt;
    int(0)
  }
  ["cache"]=&gt;
  int(0)
  ["administer_users"]=&gt;
  bool(true)
  ["name"]=&gt;
  string(7) "test"
  ["mail"]=&gt;
  string(14) "test@example.com"
  ["pass"]=&gt;
  string(3) "aaa"
  ["status"]=&gt;
  string(1) "1"
  ["notify"]=&gt;
  int(0)
  ["timezone"]=&gt;
  string(13) "Europe/Warsaw"
  ["signature"]=&gt;
  string(0) ""
  ["picture"]=&gt;
  NULL
  ["picture_delete"]=&gt;
  int(0)
  ["picture_upload"]=&gt;
  string(0) ""
  ["signature_format"]=&gt;
  string(13) "filtered_html"
  ["init"]=&gt;
  string(14) "test@example.com"
  ["is_new"]=&gt;
  bool(true)
}