You may have already noticed that site navigation style have changed on newly created SharePoint Online communication sites. Experience is currently different on sites created earlier.
Here is a quick-n-dirty PowerShell script to change experience back to old style navigation on newly created communication sites.
[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client") | Out-Null
[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.SharePoint.Client.Runtime") | Out-Null
$webUrl = "<site url>"
$username = "<username>@<tenant>.onmicrosoft.com"
$pw = read-host -Prompt "Enter password" -AsSecureString
$ctx= New-Object Microsoft.SharePoint.Client.ClientContext($webURL)
$ctx.Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username,$pw)
$ctx.Load($ctx.Web)
$ctx.ExecuteQuery()
$ctx.Web.MegaMenuEnabled = $false //Disables megamenu experience
$ctx.Web.Update()
$ctx.ExecuteQuery()
Edited 22.8.2018 17.33:
Of course you can do it with PnP PowerShell.
Connect-PnPOnline -Url <yoursite> $web = Get-PnPWeb $web.MegaMenuEnabled = $false //$false to disable, $true to enable $web.Update() Invoke-PnPQuery
Matti Paukkonen 