Something like, "... because the installed extensions will be copied from the old cluster during the upgrade."
This is still rather opaque. Without intimate knowledge of what changes have occurred in each extension I have installed; how would I know what I have to fix after the upgrade.
The point of this behavior is that you don't have to fix anything after an upgrade - so long as your current extension version works on the new cluster. If you are upgrading in such a way that the current extension and new cluster are not compatible you need to not do that. Upgrade instead to a lesser version where they are compatible. Then upgrade your extension to its newer version, changing any required user code that such an upgrade requires, then upgrade the server again.