it's a good idea to destroy the 5.1.2-SME, it is not secure.
i think your way is practicable.
but why do you go the long way around with 5.6?
install a fresh, clean 6.x and do the backup of your user-data.
maybe you have to configure your 6.x (firewall, email, ftp, ...), because many settings will not fit.
cheers