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Client's a's website is accessed via client_a, which is also the user/FTP client name client_a.
You move all of client_a's files to the domain of client_b.
client_a still "owns" these files, so client_b can't FTP over or modify them. You need to CHOWN all files and directories to client_b.
Furthermore, if you perform the cp command as root, the owner of client_b's files now becomes root. Again, you need to change ownership to the new client.