Bleaching hair always damages the hair shaft.  I'm not sure there is a way to lighten from dark brown to blonde without suffering significant damage.  Even if you survive the initial coloring, you would need to continue to lighten the roots as they grow in.  When doing this, you usually end up re-processing some of the same hair, damaging it even more.

Continuing to highlight might be the best answer - at least that does not require subjecting all of your hair to the chemical processing.

If you do decide to color/bleach your hair, be sure to condition your hair after the processing.  I'd also recommend having it done professionally rather than out of a box.