I'm working with a computer that has XP installed on a FAT formatted disc partition and I want to upgrade it to Vista. The Vista installer would not allow me to perform an upgrade install because it ...

You can format your HDD, SSD, Memory Card, etc., using these free file system management tools or FAT32 format tools. Windows File Explorer is the default option you can use to format a drive in FAT32 ...

You can't go back to FAT32 without formatting. At least not natively in Windows. Perhaps there's a third party tool somewhere that will do it. You want to go from a modern, hard linking, journalling, ...