When the ring gets stuck, it is usually due to crusty build up. As water evaporates, minerals and other stuff get left behind, and this can layer up over time. If you take a needle or safety pin and trace the ring around where the disc sits, you should be able to remove the gunk that is cementing it in place. Also, using a longer tool like a butter knife or screwdriver will give you more leverage, and that may make it easier to twist off. If you are unable to remove the disc and replace it, then yes, you may be forced to replace the whole unit.