Change Content Type of Existing Items in SharePoint using PowerShell

Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue

#Custom PowerShell Function to switch Content type of all items stored in a SharePoint library
Function Change-ItemContentType {
    param(
        [Microsoft.SharePoint.SPWeb]$Web = $(throw "Please provide the 'Web' Parameter!"),
        [string]$ListName = $(throw "Please provide the 'ListName' Parameter!"),
        [string]$OldContentTypeName = $(throw "Please provide the 'OldContentTypeName' Parameter!"),
        [string]$NewContentTypeName = $(throw "Please provide the 'NewContentTypeName' Parameter!")
    )    
 
    #Get the list
    $List = $Web.Lists[$ListName]

    #Get the new content type
    $NewContentType = $List.ContentTypes[$NewContentTypeName]
 
    #Checkin any checked out file 
    foreach($item in $list.Items)
    {
        if($item.File.CheckOutStatus -ne "None")
        {
            #check file in            
            $item.File.CheckIn("Checked In By Administrator") 
            write-host "Checked-out file checked-in on item: "$item.id
        }
    }

    #Iterate through each item in the list 
    foreach($item in $list.Items)
    {
        if($item.ContentType.Name -eq $OldContentTypeName)
        {        
            #Change the content type
            $item.File.CheckOut()
            $item["ContentTypeId"] = $NewContentType.Id
            $item.SystemUpdate()
            $item.File.CheckIn("Content type changed to " + $NewContentType.Name, 1)
            Write-host "Content type changed for item: "$item.id
        }
    }
}

#Variables for processing
$WebURL ="http://sales.crescent.com"
$ListName ="Q1 Sales Proposals"
$ContentTypeName ="Sales Proposal"

#Get the Web and List
$Web = Get-SPWeb $WebURL
$List = $Web.lists.TryGetList($ListName)

if($list -ne $null)
{
    #Call the function to add content type
    Change-ItemContentType $web $list "document" "Sales Proposal"
}