Every AAA source licensee that I worked at made large changes to the engine source. There are many AAA titles both shipped and in development that change fundam

Every AAA source licensee that I worked at made large changes to the engine source. There are many AAA titles both shipped and in development that change fundamental things about how the engine works. Look at the GDC presentation about Mortal Kombat rollback network, or its predecessor about running UE3 at 60 fps on console for some good examples. Completely avoiding engine changed isn't the reality for AAA, and I imagine it's only true for small indies. Good news is it's pretty easy to merge small engine changes.