A new magnetic phase transition, which we assign to the separation of two different spiral phases, has been observed by ultrasound studies below the Néel temperature (TN=7.8 K) in the frustrated antiferromagnet CdCr2O4. This transition renormalizes the velocity and amplitude of the transverse acoustic mode cT whereas the longitudinal mode cL is not affected. The specific heat does not show any significant change in the entropy at this transition. Furthermore, in an applied magnetic field, the mode cT exhibits extended metastable magnetostructural states neighboring the one-half magnetization plateau in CdCr2O4. By applying an exchange-striction model we can quantitatively describe the field dependence of the sound velocity below and above the one-half magnetization plateau.