How to update an existing rule with a new UrlPathMapConfig in an existing Azure Application Gateway

param (
    [Parameter(Mandatory=$true)][string]$gatewayName,
    [Parameter(Mandatory=$true)][string]$resGroup,
    [Parameter(Mandatory=$true)][string]$appName,
    [Parameter(Mandatory=$true)][string]$protocol,
    [Parameter(Mandatory=$true)][string]$appServiceHostname,
    [Parameter(Mandatory=$true)][string]$pathRulePath)

function GetApplicationGateway($gatewayName, $resGroup)
{
    $gateway = Get-AzApplicationGateway -Name $gatewayName -ResourceGroupName $resGroup -ErrorAction Stop
    
    if($null -eq $gateway)
    {
        Throw "Gateway is null"
    }

    return $gateway
}

function GetBackendPool($gw)
{
    $backend = Get-AzApplicationGatewayBackendAddressPool -ApplicationGateway $gw -ErrorAction Stop

    if($null -eq $backend)
    {
        Throw "BackendPool is null"
    }

    return $backend
}

function AddProbeToApplicationGateway($gw, $protocol, $appServiceHostname)
{
    $probeName = $appName + "_probe"
    $probeHc = New-AzApplicationGatewayProbeHealthResponseMatch -Body null -StatusCode "200-399" -ErrorAction Stop
    
    Add-AzApplicationGatewayProbeConfig `
    -ApplicationGateway $gw `
    -Name $probeName `
    -Protocol $protocol `
    -HostName $appServiceHostname `
    -Path "/health.svc/health" `
    -Interval 30 `
    -Timeout 50 `
    -UnhealthyThreshold 5 `
    -Match $probeHc `
    -ErrorAction Stop | Out-Null

    return Get-AzApplicationGatewayProbeConfig -ApplicationGateway $gw -Name $probeName    
}

function AddHttpSettingsToApplicationGateway($gw, $protocol, $appServiceHostname, $appProbe)
{
    $httpSettingName = $appName + "_HttpSetting"

    Add-AzApplicationGatewayBackendHttpSetting `
    -ApplicationGateway $gw `
    -Name $httpSettingName `
    -Port 80 `
    -Protocol $protocol `
    -CookieBasedAffinity "Disabled" `
    -path "/" `
    -hostName $appServiceHostname `
    -Probe $appProbe `
    -ErrorAction Stop | Out-Null

    $settings = Get-AzApplicationGatewayBackendHttpSettings -ApplicationGateway $gw | Where-Object { $_.Name -eq $httpSettingName }

    if($null -eq $settings)
    {
        Throw "HttpSettings not found"
    }

    return $settings
}

function UpdateUrlPathMapToApplicationGateway($gw, $pathRulePath, $backendPool, $httpSettings)
{
    $prcName = $appName + "_pathRuleConfig"

    $newPathRule = New-AzApplicationGatewayPathRuleConfig `
    -Name $prcName `
    -Paths $pathRulePath `
    -BackendAddressPool $backendPool `
    -BackendHttpSettings $httpSettings `
    -ErrorAction Stop

    $pathMap = Get-AzApplicationGatewayUrlPathMapConfig -ApplicationGateway $gw -Name "ase_rule" -ErrorAction Stop
    $pathRules = $pathmap.PathRules.ToArray()
    $pathRules += $newPathRule

    Set-AzApplicationGatewayUrlPathMapConfig `
        -ApplicationGateway $gw `
        -Name $pathMap.Name `
        -PathRules $pathRules `
        -DefaultBackendAddressPool $backendPool `
        -DefaultBackendHttpSettings $httpSettings `
        -ErrorAction Stop
}

function main() 
{
    try {

        $gw = GetApplicationGateway $gatewayName $resGroup
        $backendPool = GetBackendPool $gw
        $appProbe = AddProbeToApplicationGateway $gw $protocol $appServiceHostname
        $httpSettings = AddHttpSettingsToApplicationGateway $gw $protocol $appServiceHostname $appProbe
        UpdateUrlPathMapToApplicationGateway $gw $pathRulePath $backendPool $httpSettings
        Set-AzApplicationGateway -ApplicationGateway $gw -ErrorAction Stop
    }
    catch {
        Write-Error $_.Exception
    }
}

main