I first installed LO 5.0.5.2 x64, then upgraded to 5.1.3.2 x86 version. Everything went fine, but the old files remained in C:\Program Files\LibreOffice 5 after the upgrade (the new files were installed in C:\Program Files (x86)\LibreOffice 5 as expected).
Of course they remain, the "upgrade" you performed is a new complete installation into a different location. For the time being until an incremental update mechanism is implemented bug 54242 (and related) the Windows installer is full install package. If you'd wanted to "upgrade" in place you needed to select the 64-bit installer for download, and even then "all" it would do is overwrite the existing DLLs. If there is an issue, it would be that the "update" notification sends you to the download page, but does not adjust the landing location for the 32-bit/64-bit instance and localization already in place.
I consider it an issue that now there's an orphaned LibreOffice in the system that has no shortcuts, no uninstall function (what was there is taken over by the new version's), and the only option to get rid of it is manual deletion. I'd expect the installer to first uninstall the old version, and then install the new one. Note that it happens correctly, with no leftover files, in a 32-bit -> 64-bit upgrade, but not vice versa.
Sure, we can call it an enhancement. @Andras, any thoughts on a custom action in the MSI installer for checking existence of both 64-bit as well as 32-bit installations when installing either. Probably fine to abort the install and have the user manually remove the prior version. But not unreasonable to provide some check to prevent orphaned 32 or 64 bit installation cut off from APPWIZ.exe add/remove.
http://www.advancedinstaller.com/forums/viewtopic.php?t=30787 Aron, did it happen on Windows 7?
Yes, it's Windows 7.