Change the Subnet for Web Application Gateway 

Connect-AzureRmAccount
$AppGw = Get-AzureRmApplicationGateway -Name BSFAppGW-Staging -ResourceGroupName BSFRSG-Staging
Stop-AzureRmApplicationGateway -ApplicationGateway $AppGw
$Vnet = Get-AzureRmVirtualNetwork -Name BSFVN-Staging -ResourceGroupName BSFRSG-Staging
$NewSubNet = Get-AzureRmVirtualNetworkSubnetConfig -Name BSFAGWsnet-staging -VirtualNetwork $Vnet
$AppGw = Set-AzureRmApplicationGatewayIPConfiguration -ApplicationGateway $AppGw -Name $AppGw.GatewayIPConfigurations[0].Name -Subnet $NewSubNet
Set-AzureRmApplicationGatewayAutoscaleConfiguration -ApplicationGateway $Appgw -MinCapacity 5
Set-AzureRmApplicationGateway -ApplicationGateway $AppGw
Start-AzureRmApplicationGateway -ApplicationGateway $AppGw