Huh. So you can't reformat an encrypted SD card from an old Android device on a new one (if you just want the extra space, and don't need to recover the contents because you have them elsewhere). It just won't even offer. You can mount/unmount, decrypt (except you can't), and that's about it. Unless this is a Samsung Special removed-the-UI-from-settings thing.
But you *can* plug it into a PC and reformat it there, then put it in the new device and encrypt your new, empty extra storage space.