{
    gsub (/\\u0026/, "\\&")
    print $0
}

GNU Awk 3.1.0
Copyright (C) 1989, 1991-2001 Free Software Foundation.

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.

\u0026 Hello world!

& Hello world!

{
    gsub ("\\u0026", "\\&")
    print $0
}

\& Hello world!