The hip-hop trio had to be dropped from the Judges' Houses stage of the competition after one of the group was unable to sort out their visa problems in time.
Two groups - Triple J and Times Red - were called up to fill the gap. Triple J have since renamed themselves Union J following the addition of George Shelley to the group.
Speaking about the last-minute switches, Walsh said: "When Rough Copy left, I found it difficult to choose between two groups to fill their place. In the end I invited boyband Triple J to come to Judges' Houses, but still felt that as I'd lost such a strong group in Rough Copy that I wanted to make sure the category was the best it could possibly be.
"I asked Gary, Nicole and Tulisa if they minded if I took a seventh act. They were happy as they felt they'd got the right six acts for their Judges' Houses trips so I asked Times Red to come along and they said yes."
Union J described the changes as a "once in a lifetime opportunity".
"We're so happy to be a four piece too. The group feels so right together, it feels like we've been together for years!" they said.
Meanwhile, Times Red added: "We are beyond excited about having a second chance. We have nothing to lose so we're going all-out to impress in Vegas."
Walsh will be joined by Sharon Osbourne at the Judges' Houses stage, which he filmed in Las Vegas. The judges can only take three acts through to the Live Shows this year.
The X Factor Judges' Houses will air this weekend in a double-bill on ITV1.
Photo gallery - X Factor Judges' Houses original final 24
Copyright: THAMES TV/SYCO