The NC and WSAM applications will be upgraded automatically when the client logs in. It's not recommended that you run different versions.
I must say I find this ridiculous as it presumes auto-upgrades will always work, doesn't have policy issues, etc.
I don't have to upgrade my Cisco VPN Client every time I ugprade firmware on a Cisco PIX, ASA or VPN Concentrator--there is a large degree of backward compatibility.
So officially there is no backward compatibility? How about unofficially or practically speaking?
I don't think you can compare the SSL VPN to the Cisco VPN client. If you want to compare apples to apples, then I would compare the Netscreen Remote Client. You can upgrade the Firmware on the Firewall without upgrading the ScreenOS client (just like Cisco).