Our 4-year-old daughter's accent is English, but she uses many Americanisms in her speech. If a native listens closely enough, they are usually able to determine our child has a foreign parent.
Although I don't think my accent has changed since living here for so long, my sentence structure has, and I do speak differently to relatives when flying back to the US to visit.
In the US, I'm not necessarily more rude, but more direct. In the UK, I have to use 25% more words to express an idea/task than I would in the US. Or at least that's how I perceive the situation.
Recently I attended a London meeting with a group of American ex-pat women. One of them was from Mississippi and had been living in the UK for 20 years, and she is determined not to lose her accent. It was hilarious to hear her speak. She honestly sounded like she just stepped off the plane. Good on her.