You wouldn't need to doa thing to the system files. Just replace the model, and they'll all be right. I'd keep the Sirian lanes the same everywhere, as Ageira builds and maintains all of them. Jist make them opposite from Gallic lanes.
As to justifying the change in-RP, don't. The lanes are horizontal. The lanes have always been horizontal. Report anyone who says otherwise to your local Mental Readjustment Center immediately.