Class is a lot more than income. It's a value system, participation in different institutions of civil society, behavioral norms, language used, dietary preferences, and more. Maybe it's different in the US but here in the UK, working class with money is still working class (I don't mean this in a perjorative sense, e.g. a working class man who makes his fortune won't suddenly start eating foie gras and going foxhunting), upper class is still upper class in poverty (e.g. an artistocrat who loses her estate won't suddenly start drinking pints of bitter and going on holiday to Ibiza). It's not 'til grandchildren that a family's class really changes. Even if the middle class becomes poorer, it won't lose its values (e.g. belief in education) that created it in the first place.