If a client will ask you to change master domain name
with another , avoid to do that with WHM , if you want avoid tons of problems .
Some client used this kind of procedure (especially changing the domain name sequentially 2 times) to hide their domain name from bandwith and perhaps disk usage , and also for other purpouse that I will not explain here .
So the best way to change a domain name is :
a) Back up the client account (database included)
c) Cancel the account
d) re-create the account with the new domain name