I have virtualbox 4.3.28
installed. I'd like to downgrade to virtualbox 4.3.26
Is it possible to downgrade without uninstalling/reinstalling?
In most cases you will likely want to perform an uninstall and then an install.
> choco uninstall virtualbox
> choco install virtualbox --version 4.3.26
In almost all cases, the question is not whether Chocolatey will support it, but whether the underlying software supports a downgrade. With respect to MSIs, they don't always support downgrades (it's configurable by the author). For other installers like NSIS, InstallShield, or InnoSetup, it may not support it at all. So it's best to usually follow the practice of uninstall/reinstall to run into the least amount of issues.
If you know what you are doing and know the underlying software will support it you can try to force an install to a lower version:
> choco install virtualbox --version 4.3.26 --force
You may also need to pass --allow-downgrade
as part of the options for forcing the install of a lower version.