I need to download the macOS Sierra installer again (latest version, 10.12.6) as my latest installer seems to be corrupted. But going into the Purchased tab in the App Store does not show Download (obviously), but Open. Even if I move out the current installer, I never get the Download button ... how can I proceed?
You will need to delete the current installer. OS X applications are usually very good about keeping track of files, even when they get moved (see How does Mac OS keep track of moved files?). In this case, the AppStore is likely keeping track of the application bundle (which you would normally want it to do) when you move it.