That seems like a great idea but how do you go about it? After all, there's no "Update this project to Windows 10" option like we've had in previous versions.
You almost certainly want to keep your existing code so you can continue to provide updates to users who haven't yet updated to Windows 10.
This leaves two options:
1. Create a new project and copy files (or links to those files) into the project.
More at https://msdn.microsoft.com/en-us/library/windows/apps/xaml/mt188198.aspx
2. Copy the existing project files and modify them.
More details at https://msdn.microsoft.com/library/mt148501(v=vs.140).aspx