Are you sure that your account has permissions to do this (see the bottom of the post at https://techcommunity.microsoft.com/t5/intune-customer-success/change-the-intune-primary-user-public-preview-now-available/ba-p/1221264 for a list of roles granted this permission)?
The option is available in my test tenant for both co-managed and Intune only devices. Perhaps your tenant missed being flighted for this feature. If your account has the proper permissions, you should open a support case to validate your tenant's flighting.