The Windows 8.1 to Windows 10 upgrade using a DVD made from the Windows10 ISO file and verified, worked correctly. All the MS-3 installed programs are working.
I have not done an exhaustive test of the MS-3 system due to time, but so far things are working. It does appear to run a bit slower than Windows 8.1, but you get the new UI.
Now for the bad news. If something goes wrong with the Windows 10 64bit upgrade, and the MS-2 or MS-3 needs to be rebuilt, the customer will have to pay for the service and shipping both ways. The good news is that with both the Windows Technical Preview upgrade and Windows 10 upgrade versions, they both worked correctly after the process on the MS-3 hardware. I do not have an MS-2 chassis here to test Windows 10 upgrade, but the WTP upgrade did work correctly when I did.
Windows 7 is a totally different story. The Windows 10 upgrade keeping the original programs did not work. Will need a total OS rebuild for MS-1's that want to try Windows 10. The customer would have to pay for the upgrade service and shipping both ways.