How to add a new host to WVD hostpool?

 I am getting same error as stevenzelenko  when I tried to change the Domain UPN for deploying new hostpool in existing tenant.

New-AzResourceGroupDeployment : 1:43:36 PM - Resource Microsoft.Compute/virtualMachines 'Testpool-1' failed with message '{
"error": {
"code": "PropertyChangeNotAllowed",
"message": "Changing property 'adminUsername' is not allowed.",
"target": "adminUsername"
}
}'
At line:1 char:1
+ New-AzResourceGroupDeployment -ResourceGroupName "a0031-WVD-USE" -Tem ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [New-AzResourceGroupDeployment], Exception
+ FullyQualifiedErrorId : Microsoft.Azure.Commands.ResourceManager.Cmdlets.Implementation.NewAzureResourceGroupDeploymentCmdlet

New-AzResourceGroupDeployment : 1:43:43 PM - Resource Microsoft.Compute/virtualMachines 'Testpool-0' failed with message '{
"error": {
"code": "PropertyChangeNotAllowed",
"message": "Changing property 'adminUsername' is not allowed.",
"target": "adminUsername"
}
}'
At line:1 char:1
+ New-AzResourceGroupDeployment -ResourceGroupName "a0031-WVD-USE" -Tem ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [New-AzResourceGroupDeployment], Exception
+ FullyQualifiedErrorId : Microsoft.Azure.Commands.ResourceManager.Cmdlets.Implementation.NewAzureResourceGroupDeploymentCmdlet

New-AzResourceGroupDeployment : 1:43:50 PM - Template output evaluation skipped: at least one resource deployment operation failed. Please list deployment operations for details.
Please see https://aka.ms/DeployOperations for usage details.
At line:1 char:1
+ New-AzResourceGroupDeployment -ResourceGroupName "a0031-WVD-USE" -Tem ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [New-AzResourceGroupDeployment], Exception
+ FullyQualifiedErrorId : Microsoft.Azure.Commands.ResourceManager.Cmdlets.Implementation.NewAzureResourceGroupDeploymentCmdlet

New-AzResourceGroupDeployment : 1:43:50 PM - Template output evaluation skipped: at least one resource deployment operation failed. Please list deployment operations for details.
Please see https://aka.ms/DeployOperations for usage details.
At line:1 char:1
+ New-AzResourceGroupDeployment -ResourceGroupName "a0031-WVD-USE" -Tem ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [New-AzResourceGroupDeployment], Exception
+ FullyQualifiedErrorId : Microsoft.Azure.Commands.ResourceManager.Cmdlets.Implementation.NewAzureResourceGroupDeploymentCmdlet