Changed mine to braided myself at around 12mths old, so know fluid got changed, then at 2nd service went through usual discussion with main dealer of: brake fluid change will be £50 sir – uh no it won’t as it’s included in service plan- ah yes so it is sir. Who knows if they actually changed it. Plus can’t see how it could be classed as voiding warranty, brake lines are ford approved, would be like saying warranty is void cos you changed wiper blades for non ford items. Obviously anything changed is at your own risk, just be honest with u insurance company,