To remove concrete sealer using acid, etch the concrete surface with muriatic acid. Make sure to dilute the acid before spraying it on the surface; this makes the process overall safer. After applying the acid, scrub the concrete sealer until all the concrete is gone. Make sure to neutralize the muriatic acid before washing it off the surface completely. The concrete slab has to be completely dry and free of debris before you can cover it with the new sealer.
There are many other kinds of chemical strippers that remove concrete sealer, including caustic, solvent-based and biochemical strippers. Chemical removal works best for spot removals and generally small surfaces. If you need to remove more than several feel of concrete sealer, it is better to use the mechanical method. To remove the sealer mechanically, grind, blast or sand the coating off the concrete.
When using a sandblaster to remove concrete sealer, maintain the nozzle of the sandblaster at your shoulder level or higher to make sure you don't damage the surface of concrete that you don't wish to remove. You can always remove the remaining sealer later if you miss a spot.
The drawback of using the mechanical method is that it is dirty, noisy and requires special skills. The drawback of using chemical strippers is that they are dangerous to work with.