<?php

$data = array(
  '2011-07-29' => 39,
  '2011-07-30' => 39,
  '2011-07-31' => 39,
  '2011-08-01' => 40,
);

$keys = array_keys($data);

foreach($keys as &$key)
{
    list(,$month) = sscanf($key, '%d-%d-%d');
    $month = sprintf("%02d", $month-1);
    $key[5] = $month[0];
    $key[6] = $month[1];
}
unset($key);

$data = array_combine($keys, $data);

print_r($data);

Array
(
    [2011-06-29] => 39
    [2011-06-30] => 39
    [2011-06-31] => 39
    [2011-07-01] => 40
)