Since the new MCC is already unlinked from the original MCC, then the users of your original MCC won't have access anymore to your new MCC. However, in order to use the same OAuth2 credentials from your original MCC to the new MCC, then you need to
add the user from your original MCC (on which you have generated the OAuth2 credentials) to your new/unlinked MCC.
Let me know if this helps.