There's a command-line app called convert that should be able to do it on the fly, but the first advice anyone would give you is to back up the data first from the drive. Given that, it makes no sense to attempt it, because if you could back the data up, you could also empty the drive, delete the partition, and recreate it.
If it was me, I wouldn't attempt to convert it at all with important data on it. I'd be copying the data off first.