Edit Permission Levels with Powershell (or another tool)

Hi all,

I need to edit Permission Levels across multiple site collections and was looking to Powershell for this task (ex. change the Contribute permission level to include "Manage Lists")

This current script allows me to Add a custom permission level, but any direction on how to edit an existing permission level?

$spWeb = Get-SPWeb http://SP2010
$spRoleDefinition = New-Object Microsoft.SharePoint.SPRoleDefinition
$spRoleDefinition.Name = "Custom"
$spRoleDefinition.Description = "Can Create and Modify Items, Not Delete"
$spRoleDefinition.BasePermissions = "Base Permission 1, Base Permission 2, etc."
$spweb.RoleDefinitions.Add($spRoleDefinition)