powershell basename replace
Get-ChildItem -Path C:\temp\files -Filter *.txt | Rename-Item -NewName {$_.Basename.Replace("old","new") + $_.extension} -WhatIf -Verbose
Source: stackoverflow.com
Get-ChildItem -Path C:\temp\files -Filter *.txt | Rename-Item -NewName {$_.Basename.Replace("old","new") + $_.extension} -WhatIf -Verbose