IAS 16 – Major Overhaul

As per IAS 16 when an asset has to undergo a major overhaul, a company may estimate it based on cost as if it had been purchased at the time of purchase.

So when I have a ‘planned’ Major Replacement occurring every x years , I de-recognize the replaced part at WDV and capitalize the new part.

But is there a separate treatment for ‘unplanned’ major replacements? and if so why does the treatment differ in its logic?