Migrate classic AzureVirtualNetwork to ARM error

I'm trying to move a new customer to Azure ARM. But I get an error on the network part.

$vnetName = 'AZURECN'
Move-AzureVirtualNetwork -Prepare -VirtualNetworkName $vnetName

I get this error:

Move-AzureVirtualNetwork : BadRequest : Migration of virtual network: AZURECN failed as the address prefix 10.94.0.1/24 of local network site CN - Ziggo Amsterdam is invalid. Please fix the network configuration before trying to migrate. 

I think the IP subnet is wrong, I have to use 10.94.0.0/24 instead of 10.94.0.1/24. But how can I change that? I cannot find the right commands in Powershell to change it. It is not possible in the Portal, I have no GUI to change the subnet.

Any suggestion?