Jockey Norman Williamson, due to ride Killusty in today's Grand National, has been ruled out because of injury.
He has been replaced by Tony Dobbin on the horse owned by the wife of composer Andrew Lloyd Webber.
Williamson hurt himself in a fall on Pizarro yesterday.
Killusty could carry up to 4lb overweight following the jockey switch.
Dobbin said: “I got the call this morning from Charlie Egerton on my way down here, but I had already eaten my breakfast.
“I won’t be able to do the weight – I would think I’ll be doing around 10st 4lb.
“I’ve never seen the horse, let alone sat on him, but I’ve spoken to ‘AP’ (Tony McCoy) about him.
“He’s a very inexperienced horse, but he’s got a great chance.”