PowerShell script to upload custom list form created using SharePoint designer (EditForm, DisplayForm)

I would like to upload and associate customized version of Editform.aspx and displayform.aspx using SharePoint designer to one of my Custom List in production and make them default. I have tried using SPFiles.Add, filestream and [Microsoft.SharePoint.SPTemplateFileType]::FormPage but contents which I customized using SharePoint designer are wiped off and I see complete new version of http://ift.tt/1v9UFlD like I am missing something here? Please advise.

  function UploadFileInLibrary            
  {            
   [CmdletBinding()]            
   Param(            
   [Parameter(Mandatory=$true,ValueFromPipeline=$true)]            
   [string]$webUrl,            
   [Parameter(Mandatory=$true)]            
   [string]$DocLibName,            
   [Parameter(Mandatory=$true)]            
  [string]$FilePath            
   )                 

Start-SPAssignment -Global              
$spWeb = Get-SPWeb -Identity $webUrl             
$spWeb.AllowUnsafeUpdates = $true;            
$List = $spWeb.Lists[$DocLibName]            
$folder = $List.RootFolder            
$FileName = $FilePath.Substring($FilePath.LastIndexOf("\")+1)             
$File= Get-ChildItem $FilePath            
#$spFile = $spWeb.GetFile($File.Name)   
#$spFile = $spWeb.GetFile("http://Server:1408/sites/TestPowerShell/Mylist/DispObservation.aspx")
$flagConfirm = 'y'            
if($spFile.Exists -eq $true)            
{            
    $flagConfirm = Read-Host "File $FileName already exists in library $DocLibName, do you    want to upload a new version(y/n)?"             
}            

if ($flagConfirm -eq 'y' -or $flagConfirm -eq 'Y')            
{            
    $fileStream = ([System.IO.FileInfo] (Get-Item $File.FullName)).OpenRead()            
    #Add file            
    write-host -NoNewLine -f yellow "Copying file " $File.Name " to " 
    $folder.ServerRelativeUrl "..."            



    $spFile = $spWeb.Files.Add('DispObservation.aspx',$fileStream,$true)
    try
    {
        $List.RootFolder.Files['DispObservation.aspx'].Delete()
    }
    catch{}


    $List.RootFolder.Files.Add($spWeb.ServerRelativeUrl,[Microsoft.SharePoint.SPTemplateFileType]::FormPage)
    $List.RootFolder.Files.Add("http://nlams10824:1408/sites/TestPowerShell/Observations1/",$spFile.OpenBinary(),$true)
    $List.Update()



}               
$spWeb.AllowUnsafeUpdates = $false;            


Stop-SPAssignment -Global              

} UploadFileInLibrary "SharePointServer" "Mylist" "C:\CustomListsPOC\Forms\DispObservation.aspx"