function Reset-SPFileContentType ($WebUrl, $ListName, $OldCTName, $NewCTName)
{
#Get web, list and content type objects
$web = Get-SPWeb $WebUrl
$list = $web.Lists[$ListName]
$oldCT = $list.ContentTypes[$OldCTName]
$newCT = $list.ContentTypes[$NewCTName]
$newCTID = $newCT.ID

#Check if the values specified for the content types actually exist on the list
if (($oldCT -ne $null) -and ($newCT -ne $null))
{
    #Go through each item in the list
    $list.Items | ForEach-Object {
        #Check if the item content type currently equals the old content type specified
        if ($_.ContentType.Name -eq $oldCT.Name)
        {
            #Check the check out status of the file
            if ($_.File.CheckOutType -eq "None")
            {
                #Change the content type association for the item
                $_.File.CheckOut()
                write-host "Resetting content type for file" $_.Name "from" $oldCT.Name "to" $newCT.Name
                $_["ContentTypeId"] = $newCTID
                $_.Update()
                $_.File.CheckIn("Content type changed to " + $newCT.Name, 1)
            }
            else
            {
                write-host "File" $_.Name "is checked out to" $_.File.CheckedOutByUser.ToString() "and cannot be modified"
            }
        }
        else
        {
            write-host "File" $_.Name "is associated with the content type" $_.ContentType.Name "and shall not be modified"
        }
    }
}
else
{
    write-host "One of the content types specified has not been attached to the list"$list.Title
}
$web.Dispose()
}
Reset-SPFileContentType –WebUrl <Site URL> –ListName <Document library display name> –OldCTName <Content type to be replaced> –NewCTName <Content type to replace it with>