You can often tell if people have attempted to change their accent. It may only be the odd syllable but it will give them away.
See, I find this post really sneery. You may not have meant it like this, but there's an obvious tone of "you're not one of us, you can't keep it up, your slightly different accent shows your real breeding".
Why do you have to assume that people are trying to be something (that you consider) they're not? Can it not just be that a person has lived in a few different places and has picked up on the different sayings and ways of saying things of those around them?
I love hearing different accents, I love finding out how and why people have moved about, experienced life in different places and I think that makes someone much more interesting, not to mention brave.
In my (very professional) job/industry I get to work with people from all different areas of the UK and abroad who have moved about with work. When people from the home counties, or Italy/Germany/whatever say little Scottish phrases or pronunciations they have picked up from living/working in Scotland I don't think to myself "pfft, you are trying to be Scottish, who do you think you are trying to be one of us".
I find the sneering very unbecoming.