Same as any other drive. Copy content to another location, stop array, change disk format, start array, select the checkbox beside the option to format the drive, make sure that specific drive is the only one listed as needing formatted, then select format. When that's done, copy the content back in place from wherever you put it.
Since this is the ReiserFS to XFS thread, I'll assume you have a single cache disk that you want to convert to XFS. If you select multiple cache slots to be visible, you will lose the ability to format the cache drive in anything but BTRFS, so don't do that.
It's also a good idea to be sure that your data copy method and destination support sparse files and symlinks, as there are likely to be a bunch of both things on a well used app / vm / cache drive.