It's usually the people who move in (or not moving in) who change neighborhoods. It's the internal migrants, like in North Dakota, or immigrants or professional class who change neighborhoods for the better. This is why poor areas _want_ "gentrifiers" to come in to invigorate places in stagnation or decline. It's atypical for improvement to be grass roots.

Anyone who has lived in a typical urban core from the 90s knows that poor neighborhoods don't just start having cleaner streets, more shops and services out of the blue displacing liquor stores and repair shops and weeds.