9mm doesn’t sound much but percentage wise, that’s quite a difference in the inset.
Safety wise, clearance and geometry will be altered, the important bit is whether or not the new wheel & tyre combo is within design tolerances for that model range. If the difference causes the tyre to rub the arch under suspension compression or the inner wing on full steering lock, or if the wheel could foul the brake caliper during cornering, then that would be just a little bit iffy to say the least. Suspension, steering, stub axle and wheel inset / offset are designed to work together to give steering weight, feel, predictability and handling stability. Changing one of those will alter the geometry, again it depends whether that change is within design tolerances.
Insurance company will wash their hands if an alteration from original spec is not declared or within design tolerances.