How to update Multi-Line field of SharePoint Add-In List to Enhanced RichText using PowerShell

Question

I have a List column in SharePoint Add-In in SharePoint Online which is a Multi-Line Rich Text field. I want to convert it to Enhance Rich Text field using Power-Shell.

$URL, $Username, $password and $ListTitle are passed as parameters to the function in which this code is written.

try
{
    $ctx=New-Object Microsoft.SharePoint.Client.ClientContext($Url)
    $ctx.Credentials = New-Object Microsoft.SharePoint.Client.SharePointOnlineCredentials($Username, $password)
    $ctx.Load($ctx.Web)
    $ctx.ExecuteQuery()
    $ll=$ctx.Web.Lists.GetByTitle($ListTitle)
    $ctx.Load($ll)
    $ctx.ExecuteQuery()
    #Add new field to the list
    Write-Host "`nGot List" $ListTitle

    $fieldColl = $ll.Fields;

    $isDescriptionPlainText = $false;
    $ctx.Load($fieldColl);
    $ctx.ExecuteQuery();
    foreach ($item in $fieldColl) 
    {
        if($item.InternalName -eq "VisualDescription")
        {

            if($item.InternalName -eq "VisualDescription" -and $item.RichText -eq $false)
            {
                Write-Host ("`n'{0}' Field available.Making Rich Text"-f $item.InternalName)
                $msg=("`n'{0}' Field available.Making Rich Text"-f $item.InternalName)
                writeToLog $msg
                $isDescriptionPlainText=$true;
                $item.RichText = $true
                $item.update()
            }
            else
            {
                Write-Host ("`n Field not available or already a rich text.")
                $msg=("Field not available or already a rich text.")
                writeToLog $msg
            }
        }           
    }
    if($isDescriptionPlainText)
    {
        $ll.update()
        $ctx.Load($ll)
        $ctx.ExecuteQuery()
    }
}
catch
{
    $errorMessage = $_.Exception.Message
    $errLineNumber = $_.InvocationInfo.ScriptLineNumber
    $errOffset = $_.InvocationInfo.OffsetInLine
    $msg = "The script failed due to an unexpected error. [Reason]: $errorMessage [Error Line Number]: $errLineNumber ($errOffset)"
    writeErrorToLog $msg
    # Log the entire stack trace
    writeErrorToLog $_.Exception
    Write-Error $msg
}

I cannot use $item.RichTextMode = "FullHtml" as RichTextModeproperty does not exist $item

Answer 1

After struggling for few days I finally was able to solve the problem of updating Multi-Line field of SharePoint Add-In List to Enhanced RichText using PowerShell.

We need to make changes in SchemaXML property of the field and replace RichTextMode from Compatible to FullHtml

$item.SchemaXml=$item.SchemaXml.Replace("RichTextMode=`"Compatible`"","RichTextMode=`"FullHtml`"")