To be clear, Onboard is intended for non-domain devices that you cannot get a certificate on via domain autoenrollment. If you are migrating domain devices to a new domain, you can configure autoenrollment on that domain to give those devices user certificates to a new domain.
If these are NOT domain devices and you migrate to a new domain, will devices NOW be added to the new domain? If those devices will not be added to the new domain, you can still use to same onboard certificates on those devices to authenticate them via ClearPass. Please let us know your migration details...