The SKU isn't modifiable, https://learn.microsoft.com/en-us/azure/virtual-machines/update-image-resources?tabs=cli%2Cpowershell2%2Ccli4#update-resources
But best bet would be to create a new definition: https://learn.microsoft.com/en-us/azure/virtual-machine-scale-sets/tutorial-use-custom-image-powershell#create-an-image-definition