Noel Fehily has picked up the spare ride aboard leading Coral Welsh National contender Upswing.
With owner JP McManus' retained jockey Barry Geraghty riding elsewhere, connections have moved to secure Fehily's services for the Jonjo O'Neill-trained seven-year-old, who is a general 8-1 shot for the Grade Three highlight.
O'Neill said: "We knew that Barry wasn't going to be there, so we had to look for somebody else that was going to be available to ride at Chepstow, which Noel is. Although he has not sat on the horse lately, he sat on him a while back and was in schooling with him."
Upswing will be the sole contender for McManus in Sunday's extended three-mile-five-furlong affair after trainer David Bridgwater ruled out Wyck Hill following a lacklustre piece of work on Wednesday morning
The mud-loving 11-year-old, who would have been making his first start since pulling up in this year's Crabbie's Grand National at Aintree, will now be aimed at the Eider Chase at Newcastle in February, a race he won in 2014.
Bridgwater said: "The conditions at Chepstow would have been ideal but Wyck Hill is not going to run.
"He didn't work particularly well this morning and I was quite disappointed. I am gutted but these things happen in racing.
"He will go for the Eider Chase now, but we will probably find a race before it. It may be another novice hurdle race as he went close to winning one last year before running in the Eider."