Through Eclipse I can easily re-install (without having to uninstall the app first), a small app I am currently developing, any change any number of times.
When I export that app ("release mode") and place it on a website, it downloads and installs without any problem. (I have to check "Unknown sources" for allowing installation of non-Market apps first, of course).
If I uninstall this app, then download it again and install it again, it installs without any problem.
However, if I try to re-install this app without uninstalling it first, then Android notifies that the installation failed.
My question is why? What makes an Android app re-installable? (i.e. without having to uninstall it first, thus losing its existing settings)
You cannot replace a signed application with an unsigned one.