Why not use windows format?
To the best of my knowledge it can't be done whilst preserving the data. I wouldn't personally trust my data to a unproven third party utility. Back the data up and reformat the drive.
Why do you need to convert the drive? There's lots of limitations with FAT32 such as maximum file sizes and slack space due to large cluster sizes.
Try fat32format - there's a GUI version if you don't like the command line, but it's pretty simple in itself (from an administrative command prompt: "fat32format x:")
http://www.pendriveapps.com/format-fat32-larger-than-32gb/
This app. reckons to be able to convert up to 2TB external drive.
This seems to work OK, although the interface isn't the most intuitive I've ever used.