Depends on what updates you do. Updating firstname, lastname, email, groupname, … won't impact the behaviour of a user in the system. If you change the membership of a user in a group, this might impact FUTURE processes, as the user won't be able to claim tasks anymore for the group he/she is removed from. But the existing processes that involve that user remain unaffected. When a user/group is deleted, existing processes which reference the user remain unaffected. Offcourse, if you don't have "userX" anymore, all tasks assigned to "userX" will still exist. It's your responsibility to locate them an delegate them, for example, or cancel those workflows…