It's not as simple as applying a blonde coloring to dark hair. Instead, you must permanently bleach the dark color out of your hair to convert to blonde. But as it grows out, the roots will be your dark natural brown. That means that to maintain your own hair to match the system (which will not have dark roots), you'll need to repeatedly bleach your own hair as it grows out. Eventually your own hair will grow weaker due to all the bleaching. I'm sure you've seen women who bleach their hair too much.

As for your hair system, bleaching the hair weakens it tremendously too. If you're certain you want to do this, the system should probably be purchased blonde to start with.

Overall a bad idea, really. But if you're intent on becoming a blonde, you could always use a blonde full cap system instead of a partial piece. Shave your own hair off, and you have no maintenance issues other than periodic service to the full cap.