I use lot of asset bundles in my game and I use hash values to manage asset bundle versions.
I've confirmed that asset bundle hash values being changed after updating unity.
is there a way to keep fixed hash values with different unity versions? (I don't want to let the user to download all asset bundles every time when I update unity).
Different Unity version will generate different hash values, as you will use a different engine version to create your assetbundles. That is totally fine, allthough you might have assetbundles which will also work with different engine versions without building new assetbundles (e.g. if you have assetbundles only containing text files).
If you want to manual controll of downloads of assetbundles you can switch to use the version or the CRC code, which is generated when building assetbundles (not sure if that doesn't change on an engine version change though).
I'd recommend to keep an Unity version installed with which you build your latest client and build your assetbundles with that version. It's not the best workflow, but for platforms which don't support downwards compatible assetbundles (iOS), you don't have much of a choice.
