Break off the tabs with a pair pliers. You will then be able to use a small screw driver to pry off the back cover.
I used a needle nose pliers to carefully break off as much plastic as I could from the bottom. (Shown by the arrows.)
I carefully broke away the plastic from the top with a needle nose pliers.
After I removed as much plastic as I could, the piezo disk was easy to remove from the remainder of the plastic. I did this with all the piezos with the same results. No ruined piezos.