A knee injury ruled Fletcher out of the Games and Williams will now take his place in the U73kg category.
"I'm very happy at being given the chance to fight at the Commonwealth Games," said Williams, who was a member of the Team GB judo squad at the London Olympics two years ago.
"Whilst it's not the perfect preparation, I'm always in fighting shape and I certainly believe I can challenge for a medal. I'm really looking forward to getting up to Glasgow."
The judo takes place in Glasgow between July 24-26 next week.