migrating VFAT to a journaling FS without moving its data away possible?

[Rava]

Is migrating a VFAT partition to a journaling FS (e.g. ext3 or NTFS) without moving away its data possible?
My largest 2 1/2" external Usb harddrive has its largest partition as VFAT. I need to change that into a journaling FS that supports symlinks - like ext3 - but without the need to move away its data first.
Is there a way to archive this with free Linux software?

The reason why is because I currently do not have the free capacity to temporarily store all this partition data.
(The most crucial data on it is already either a backup from another drive, or is also available elsewhere, so even when there would be a data loss during the migration it would not be that bad a thing)