I do not believe you can safely and reliably convert NTFS back to FAT32 without removing the partition and making a new one.
http://faq.arstechnica.com/link.php?i=1820
Partition Magic allows you to convert an NTFS partition back to FAT32, but there is a reason that Partition Magic is not so lovingly referred to as "Partition Tragic". Often conversions simply fail, or worse, destroy data.