Is it possible to upgrade an Android OS in Virtual Box to the next major version?
I've installed Android v7.1, but want to upgrade to v9.
I've been unable to find an ISO for v9, so I wonder if there is an internal upgrade process I can use.
You don't find an Android x86 9.0 image because now (2019-03-03) they are still working to get the Android 8.1 release finished.
As far as I know the Android x86 project does not provide the possibility to upgrade an existing installation. The recommended way to get a new version whilst keeping the existing app data is to use two partitions, one system partition and one data partition.
Therefore an "upgrade" can be done by creating a new virtual machine, with an empty system partition and the existing data partition for the old installation: