I won’t go into the reasons why you’re doing that. Instead, I’ll just focus on your plans, because I don’t think a gradual change of accent will go unnoticed. Just do it. Embrace the American accent you love so much and live it to the fullest. And if anyone asks you what happened, just say you’re tired of your own accent and are trying something new. Explain it clearly to others and own up to it. This will make you feel good because you’re being honest by telling others why you have decided to change.