1. They lose their accents.
I never had much of a strong accent in the first place, having been raised in Dallas, but after two years in Japan, one in Korea, and several across Europe, Oceania, and South America, even old friends are a little surprised how my twang has flattened out. The downside of this isn’t losing the accent, but constantly explaining to non-Texans and Texans alike why you don’t have one.