My family moved from New Haven to San Francisco when I was 11. We were told we all had New York accents when we got there. I couldn't tell, of course. seven years later I was at a California Aggie school (UC Davis) and immersed in folk music. My brother told me I had developed quite a country twang. I lost it as I moved back to the Bay Area (Berkeley). I visited the old neighborhood in Connecticut some 10 years later, and was shocked at all my neighbors' strong New York accents.
When I visited Ireland on business in the 90's, I recall having a sort of out of body experience, as I was complaining on the phone to a computer tech support line back in the USA. I could hear my own flat California-American accent echoing through the cubicle area, in start contrast to the gentle tones of my Irish co-workers.
Nowadays I don't really notice other Americans' accents that much. They seem much more subtle these days than they did some 30 years ago. I think with TV/Radio/Films people have been gradually losing their regional accents, and it's all becoming more or less one American accent.
It is interesting to hear various Germans speak English. The ones from Frankfurt seem to have an American accent. I guess the Munich crowd sports the ersatz-British accent.
The only time I think I reverted to an East Coast accent was when my jaw was fractured and I started dropping my R's like a Bostonian. Fortunately that healed enough so I could sound like a Californian again.
Like most people, I think, I tend to adopt the accent of the location where I'm staying for a long time. I think when I came back from only about a month in Ireland I had a slight lilt to my cadence. But that also soon passed.