I just tried to re-install Piwik and re-use existing tables and got a "Method does not exist on SitesManagerAPI::getInstance()->getUsers()" in Installation/Controller.php
This seems to have happened quite some time ago, so maybe we should create tests for this case.
In 356d476: Fix Installation when trying to reuse existing tables
Cheers. Tests for installer wished in: #2935
Fix Installation when trying to reuse existing tables