Upgrade Android to a new major version in Virtual Box
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:

https://groups.google.com/forum/#!topic/android-x86/5XrE2lo7uac