It comes as Amanda Holden, who is currently playing the role at the Theatre Royal Drury Lane, announced this week that she is three month’s pregnant.
Holden will leave the show on October 3rd before Walsh starts two days later.
The popstar was due to reprise the role of the feisty princess in December as she continues to carve out an acting career during the Girls Aloud hiatus.
She has recently been seen in the children’s comedy movie Horrible Henry, while her fellow bandmates have gone on different pursuits.
Sarah Harding has also pursued the acting route, while Nicola Roberts is releasing her debut solo album. Cheryl Cole became a judge on the X Factor, while Nadine Coyle has opened a string of Irish pubs in the US.
This will not be Walsh’s first foray onto the West End stage, however. She started her career at the age of 12 playing Cosette in the musical Les Miserables.
Once her stint ends, she could be set to reunite with her fellow bandmates, after it was hinted that a ten-year reunion tour could take place in 2012.
Meanwhile, 40-year-old Holden announced her delight at becoming pregnant after losing her baby in February, seven months into her pregnancy.
She wrote on her official blog: “My first priority is of course my family. I will therefore be adjusting one of my professional commitments accordingly.
“Due to its physical content this means I will end my contract with Shrek The Musical eight weeks early. I will endeavour to continue the show until the 3rd October. I do however ask you to bear with me as I will be putting myself and this pregnancy first.”
Nigel Lindsay, Nigel Harman and Richard Blackwood will remain in their roles on the show.