FAT32 > NTFS requires no reformate it can be done on fly but make back ups!
NTFS > FAT32 requires a full format
The drives in your system can be a mixture of NTFS & FAT32, the only problem is if you try to put the NTFS drive into an older pc with older o/s (ME or below)
So you can leave the external disc as it is and will work fine!
PS i have a NTFS Disk with windows, FAT32 External USB & 1 internal ide hdd!