I've seen people mention a rename command, but it is not routinely available on Unix systems (as opposed to Linux systems, say, or Cygwin - on both of which, rename is an executable rather than a script).  That version of rename has a fairly limited functionality:
rename from to file ...
It replaces the from part of the file names with the to, and the example given in the man page is:
rename foo foo0 foo? foo??
This renames foo1 to foo01, and foo10 to foo010, etc.
I use a Perl script called rename, which I originally dug out from the first edition Camel book, circa 1992, and then extended, to rename files.
#!/bin/perl -w
#
# @(#)$Id: rename.pl,v 1.7 2008/02/16 07:53:08 jleffler Exp $
#
# Rename files using a Perl substitute or transliterate command
use strict;
use Getopt::Std;
my(%opts);
my($usage) = "Usage: $0 [-fnxV] perlexpr [filenames]\n";
my($force) = 0;
my($noexc) = 0;
my($trace) = 0;
die $usage unless getopts('fnxV', \%opts);
if ($opts{V})
{
    printf "%s\n", q'RENAME Version $Revision: 1.7 $ ($Date: 2008/02/16 07:53:08 $)';
    exit 0;
}
$force = 1 if ($opts{f});
$noexc = 1 if ($opts{n});
$trace = 1 if ($opts{x});
my($op) = shift;
die $usage unless defined $op;
if (!@ARGV) {
    @ARGV = <STDIN>;
    chop(@ARGV);
}
for (@ARGV)
{
    if (-e $_ || -l $_)
    {
        my($was) = $_;
        eval $op;
        die $@ if $@;
        next if ($was eq $_);
        if ($force == 0 && -f $_)
        {
            print STDERR "rename failed: $was - $_ exists\n";
        }
        else
        {
            print "+ $was --> $_\n" if $trace;
            print STDERR "rename failed: $was - $!\n"
                unless ($noexc || rename($was, $_));
        }
    }
    else
    {
        print STDERR "$_ - $!\n";
    }
}
This allows you to write any Perl substitute or transliterate command to map file names.
In the specific example requested, you'd use:
rename 's/^/new./' original.filename