Replace document properties using powershell

Question

Hello ,
I have document library. It has one column of categories. There are multiple categories tagged for many document .I want to replace some old categories with new one using powershell.
In other words, first i need to check if that item column has that property if yes replace it with new property. 
I dont want to change properties manually as this need checking out document. 
Please guide. powershell script will be helpful for me.

Answer 1

http://blogs.msdn.com/b/paulking/archive/2011/10/04/using-powershell-scripts-to-modify-document-properties-in-sharepoint-2010.aspx
# Load SharePoint library
[system.reflection.assembly]::LoadWithPartialName("Microsoft.Sharepoint")
# Connect to the site collection http://SP2010 and store the object in the $site variable
$site = New-Object Microsoft.SharePoint.SPSite("http://SP2010")
# Connect to the root site in the site collection and store the object in $root
$root = $site.rootweb
# Store the Shared Documents document library in a variable $Docs
$docs = $root.lists["Shared Documents"]
# Display all the documents, their titles, names and IDs
$docs.items | format-table -property title,name,id
# Updates the title for each item in the list with Name
$docs.items | ForEach { $_["Title"] = $_["Name"]; $_.Update() }
# Display all the documents, their titles, names and IDs
$docs.items | format-table -property title,name,idOr try http://sharepoint.stackexchange.com/questions/78084/how-to-update-document-meta-data-using-powershell$listItem = $web.GetListItem($file.ServerRelativeUrl);
$listItem.File.Checkout();
$listItem.Properties['My Cust Prop 1'] = 30
$listItem.Properties['My Cust Prop 2'] = '2012-10-25T00:00:00Z'
$listItem.Update();
$listItem.File.Checkin('Updated')http://sharepoint.stackexchange.com/questions/61159/get-document-properties-keys-valuehttp://social.technet.microsoft.com/Forums/ro-RO/db9191e4-c1b6-4d4b-aa58-a352da1839fc/powershell-script-to-create-document-libraries-with-specific-properties?forum=sharepointadminprevious
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