@all:
The update of Pages has nothing to do with existing Pages documents, wherever they are stored. Only the application is changed and that is not in any user area.
Pages documents that were previously saved would automatically have autosave enabled, provided these documents were saved to a filesystem where autosave is supported. If not, Pages will throw a dialog telling you this. If you ignore this warning, you had better frequently use a manual save or the unsaved changes will be lost when quitting Pages or worse, a power outage.
With regard to the previous paragraph, for those that foolishly leave unsaved Pages documents open and then proceed to update Pages, I have no idea what would happend to that document, since I certainly don't do this. Consider it caveat emptor and not Apple's fault if it whacks the open document and changes are lost. My rule of thumb is to immediately and manually save a new document before any content is added.
The user is responsible for keeping track of where their documents are stored. Remember, that Pages will continue to store revised as well as new documents in the last saved folder location — until you change it.
If your Pages documents are on Spotlight indexable media, then you may be able to locate missing documents by using the Spotlight menu bar tool 🔍, a Finder Window search field, or even the Dock's App tool with cmd+2 to open the Files panel. Then you can do some of the following, where strings are case-insensitive, and < or > mean before or after when used with dates:
kind:pages created:12/01/2025-02/22/2026
kind:pages name:partialname font:baskerville
kind:pages modified:>02/01/2026