VSys One can use one of two distinct security modes.
While the users and their basic attributes (user ID, password, Active Directory account, password expiration, enabled, superuser, tool profiles) are compatible between the two different modes, nothing else is. If you're already using VSys in compatibility mode then you'll have to re-define each user's security rights as well as define default user rights and security roles to move to advanced mode. (You can do this while compatibility mode security is still enabled and without affecting the existing compatibility mode rights of those users.) When you switch to advanced mode, the new rights kick in and the old ones are ignored.
For now, if you are using Compatibility mode you do not need to upgrade to Advanced mode, although Compatibility mode will be removed in VSys One in Version 4.0. If you're starting out with VSys or just enabling VSys Security, you should use Advanced mode.