I don't plan on developing an "english" accent. Though I make fun of my accent, I'm fine with it and I think that anyone that tries to change their accent after living somewhere for a short period of time, ie Madonna is absolutely ridiculous.
Over time, accents can and do change. My dad has been living in America for 20 years and his Irish accent, which was once quite strong, is nearly undetectable now. On the other end, I have family who moved to Ireland as teenagers, have lived there for 15 years and now have Irish accents. Their accents aren't fake or put on. It does take considerable time, but it does happen.