when copying an account from another server, make the db backup the last step
I always use the express transfer feature to prevent the site from being accessed on the old server, but some sites are just too busy that loss of data can happen if the copy takes too long.
Given the following scenario:
e-Commerce with database and 50GB homedir.
If you are on a local network this can take at least 30 minutes.
So my suggestion is, after locking the user on the old server, copy the databases.
This way there is no way for data being on the old database.