And Amanda didn't take the news too well...
Simon wound up Amanda in a prank phone call on Nick Grimshaw's BBC Radio 1 Breakfast Show, saying she would no longer have a full-time job on BGT.
Music mogul Simon said 31-year-old Cheryl would be replacing her for three days a week because she is "younger".
Poor Amanda fell for it hook, line and sinker. The 44-year-old was not impressed at all, snapping back: "No she's bloody not!"
Amanda was put out of her misery and at the BGT press launch later that evening, and we bet she's secretly plotting her revenge on Simon...
She told a group of reporters: "I have to say I felt really bitter about Cheryl, and she knew nothing about it."
Meanwhile, prankster Simon said: "I told her she was being replaced for three days a week by Cheryl on the live shows because she is younger and knows more about music."
The prank will be broadcast on Nick's radio show today.
Britain's Got Talent returns to ITV on Saturday 11 April at 8pm.
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