What does it take to switch to a new ATC? AFAIK they were in the process of switching to the Vietnamese ATC and in fact the last message was a "good night" to t

What does it take to switch to a new ATC? AFAIK they were in the process of switching to the Vietnamese ATC and in fact the last message was a "good night" to the Malaysian ATC. Could the process of switching to the new ATC mean that they were left without communications in the deciding minutes?