Transferred from Other Hardware I finally have sufficient HDDs to transfer over backups. Can I change from mbr to gpt on two of the drives to avoid transferring 5GB of data just to change the disc ...

The recommended method to convert these disks is to upgrade the operating system to Windows 10 first, then perform the MBR to GPT conversion. So 2k8 sounds like a no no even for an offline conversion.

You can convert an MBR disk into a GPT disk using the Windows interface. After backing up or moving the data to a GPT disk, right-click each partition and select Delete Partition or Delete Volume.