Sharepoint online - powershell remove choice from list field

I have a few hundred sites in which I have a list (doclibrary) where I use a choice field which has 8 choices. Now I need to delete a choice and rename (or delete and add a new one) another choice. I created this powershell script sofar:

$username = "blabla"
$password = "bla"
$url = "https://blabla.sharepoint.com/"

$securePassword = ConvertTo-SecureString $Password -AsPlainText -Force

Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.dll"

Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Runtime.dll"

Add-Type -Path "c:\Program Files\Common Files\microsoft shared\Web Server Extensions\15\ISAPI\Microsoft.SharePoint.Client.Taxonomy.dll"


$ctx = New-Object Microsoft.SharePoint.Client.ClientContext($url)
$credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($username, $securePassword)
$ctx.Credentials = $credentials

if (!$ctx.ServerObjectIsNull.Value)
{
    Write-Host "Connected to SharePoint Online site: '$Url'" -ForegroundColor Green
}

$rootWeb = $ctx.Web
$sites  = $rootWeb.Webs

$ctx.Load($rootWeb)
$ctx.Load($sites)
$ctx.ExecuteQuery()

foreach($site in $sites)
{
    $ctx.Load($site)
    $ctx.ExecuteQuery()

	Write-Host $site.Title "-" $site.Url

	$olist = $site.get_lists().getByTitle('CustomList');
	$fieldColl = $oList.Fields;
	$ctx.Load($fieldColl);
	$ctx.ExecuteQuery();

	foreach ($fieldTemp in $fieldColl)
		{
 			if ($fieldTemp.InternalName.Contains("QuestionType"))
				{
        			Write-Host $fieldTemp.InternalName
					$FieldTemp.choices
					$FieldTemp.choices.remove("General")
					$FieldTemp.update()
        		}

		}

} 

On the remove command I get an error "Collection was of a fixed size".

What can I do to achieve my goal?

Thanks, Mike