Well the only thing that should affect the speedo reading is the outside diameter of wheel plus tyre. So, if you get bigger wheels but
lower profile tyres you will have the same outside diameter and the speedo will be unaffected.
Saying that I reckon the affect on the speedo from changing to a larger diameter would be negligible. I'm sure there's plenty of folk on here who have done the same and can confirm whether it makes any difference.
Remember you speedo is nowhere near 100% accurate anyway.