“Fresh start” got it working. Thanks for troubleshooting help.
New XPS 15 9570 with Windows 10 pre-installed works fine. Upgraded hard drive and attempted to (re)activate Win 10 without success. Have attempted recovery USB disk from original HD and new Win 10 ISO. Have also atempted to use the troubleshooting tool “after hardware change” using my MS account that was set up from original working Win 10 activation. Although the previous working computer (original HD) is shown, the system indicates that I cannot use it to reactivate Windows. Call to Dell support was very frustrating. They told me they could fix it if I upgraded to premium support for $260! I might as well buy a new copy of Win 10. Is that Dell’s business model now? Why make the HD upgradeable? Any help from the community before I shell out $ for a new copy of Win 10? Thanks in advance. David  
Win 10 Reactivation after Hard Drive Upgrade