Let's do some simple steps if we've to transfer an account (suppose that we're transferring is the account ACCOUNT_M from SERVER_A to SERVER_B).
1) 48 hours before transfer the account
Reduce TTL (Time To Live) of the ACCOUNT_M at SERVER_A
vi /etc/namedb/zones/ACCOUNT_M
Change "$TTL 3600" to "$TTL 300"
Restart namebd ("rndc reload")
2) Go to destination server (SERVER_B) and fill the Import User fields.
After the "ImportUser" is done from the remote, at SERVERB set for that user/site:
* IP on the new server
* DNS servers on the new server
3) Go to SERVER_A and modify main IP for the site/user transfered to the new IP