Get-ChildItem -Filter "468_*" | ForEach-Object {
    $newName = $_.Name -replace "468_", "628_"
    Rename-Item $_ -NewName $newName
}

C:\Users\jmabe\Documents\cst\cst.cat\assets\cstimage

C:\Users\jmabe\Documents\cst\cst.cat\assets\cstvideo