https://github.com/PowerShell/PowerShell/blob/master/build.psm1#L2071

We need to consider what combinations of productCode and upgradecode needs to be changed for 6.1

As @bergmeister noted, ProductCode is automatically changed:

... the productCode is always a new Guid when being called fromStart-PSPackage higher up here.

See the docs about preparing an upgrade: https://msdn.microsoft.com/en-us/library/windows/desktop/aa370837(v=vs.85).aspx