PreBuild event to remove xaml files in main project

I have a group of xaml files in a Shared Project. At build time, I want the xaml files to replace the files in the Main Core project for Xam Forms.

Basically, I am trying to override xaml files. I'm using a viewModel, so I only need to change the xaml. If a client needs a different UI for a screen I'd like to just replace the xaml with a Shared Project.

Any Ideas?