I just barely did this yesterday on Qlik Sense 3.1; it totally works. Like @Michael instructed, I used pgAdmin3, but I used the Repository Db Superuser Password from the installation process to connect. And, yes, you absolutely must bounce the Windows services for Qlik Sense (since the Central Proxy has to be restarted, and you can't get into the QMC yet).
Though, in my case, after bouncing the Repository Database (because it makes all the other services bounce too), many of the Qlik Sense services failed to restart automatically, so I had to manually start them; watch for that. Once that was done, I got into the QMC just fine. (And then I immediately change the default Virtual Proxy's "Windows authentication pattern" setting to "Forms", because I also hate Http Basic Authentication's inability to logout without closing down the browser.)
Now that I know how to do this, I find it easier than having to remember to stop the IIS website instance that's using ports 80 and 443 before I start the Qlik Sense installation. I'm also glad that I have a clean install of Windows, because Qlik Sense uses a ton of ports. I'm a little nervous that at some point, I'm going to have once again use netstat to manually confirm that there's no port conflicts for other services while troubleshooting a problem. E.g.: netstat -a -b -n -o | find "4242"
