Your Android 8 devices should be compatible with the SHA-2 certificate but there is process to follow when swapping over root certificates that are binded to the DS, the certificate cannot be changed instantly.

 

When the new certificate is generated the current enrolled devices need to check in order to learn about the existence of the certificate before the certificate can be swapped over.

 

I would recommend that you to raise a support ticket in order for a technical support agent to assist you in changing over the certificate correctly.