Change Content Type of Existing Items in SharePoint using PowerShell

Requirement:
We've an existing document library with 100's of Sales proposal documents in it. To enforce standardization, sales team created a content type "Sales Proposal". We got a requirement to change content type of all existing documents stored in a particular document library with the default content type "Document". In other words, All the documents now need to be assigned a new content type "Sales Proposal".

How to change content type of existing Items (documents) in SharePoint?
You can change the content type of the existing items by getting into edit properties page of the particular item and switching "Content Type" field.
While its relatively easy and straightforward to change content type of existing items in SharePoint, you do not want to go through each and every single document in the document library and manually assign the new content type to it, as it will be time consuming, isn't it?

Well, let us use PowerShell to make it quick and easy! This script iterates through all documents stored in the given document library, checks for the content type of each document and if it is a specific content type , it  replaces old content type with the new content type.

PowerShell script to change item content type in SharePoint

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"
}

This will change the old content type "document" of all documents to a new content type 'Sales Proposal'. Next time, when users edits the documents, they'll be prompted to enter required fields from the new content type.

Here is my another post PowerShell for SharePoint Online to Change Content Type