Hello Millar,
Did you try to switch to pure-ftpd through WHM? GOTO-- >> FTP configuration -->> switch to current FTP server to the other one.
Also, if that didnt work,
You should try changing the cpanel version from stable to release in your /etc/cpupdate.conf
and run /scripts/upcp --force and then try to switch the FTP server.
That should do the trick.
Anush Kumar