With an increasing number of users, there should be an increasing number of test cases. By adding new test cases for new features and new bug-fixes, over time there are many tests and the acts of doing another release or changing internals become less dangerous. The test cases combined with many releases over a longer time, can slowly build up a trust-level that the basics work. Doing subsequent releases is then less of a nerve-wracking thing. Adding another billion users should be fine. No need to worry.