Sociolinguistics 5, Exercise 3: New York non-rhoticity

In the 18th century, some eastern and southern US port cities with British
ties (New York, Boston, Charleston) became non-rhotic among the upper class.
British non-rhoticity was the prestige form, so imitating it carried prestige.

Non-rhoticity stayed prestigious in the US until the 1860s Civil War shifted
power to rhotic areas. Rhoticity then gained prestige nationwide, most
decisively after World War II (Labov, Ash and Boberg, 2006). By the late 20th
century, non-rhotic accents in US eastern and southern cities were often seen
as working-class or immigrant, while rhotic accents were seen as "General
American".

What changed: which region held power, and so which form counted as
prestigious. The sound itself did not change; its social meaning did. Source:
Wikipedia, Rhoticity in English, and Sociolinguistic prestige.