Hyper-V on Windows 10 (1903) - how to set default configuration version?

I have upgraded my Windows 10 from 1809 to 1903.

I have several VMs running under Hyper-V with configuration version 9.0.

Using the command Get-VMHostSupportedVersion I receive the following output: 

Name                                                      Version IsDefault
----                                                      ------- ---------
Microsoft Windows 8.1/Server 2012 R2                      5.0     False
Microsoft Windows 10 1507/Server 2016 Technical Preview 3 6.2     False
Microsoft Windows 10 1511/Server 2016 Technical Preview 4 7.0     False
Microsoft Windows Server 2016 Technical Preview 5         7.1     False
Microsoft Windows 10 Anniversary Update/Server 2016       8.0     False
Microsoft Windows 10 Creators Update                      8.1     False
Microsoft Windows 10 Fall Creators Update/Server 1709     8.2     False
Microsoft Windows 10 April 2018 Update/Server 1803        8.3     False
Microsoft Windows 10 October 2018 Update/Server 2019      9.0     True
Microsoft Windows 10 April 2019 Update/Server 1903        9.1     False

So there is a new 9.1 configuration available, but it's not set as default. How can I change to 9.1 as default?

I am also unable to upgrade my existing VMs from 9.0 to 9.1, using Update-VMVersion <vmname>.

Windows tells me that I am already on the maximum supported version for my VM, so why is there a 9.1 config if it is not available for an upgrade?

Please help!