I have tested the upgrade from v6.9 unofficial to v7.1 unofficial with a Pixel 7 Pro without issue and without data loss. But it does need to be upgraded using Recovery with adb sideload and manual acceptance of the new build signature key, as with all the unofficial iodéOS builds hosted by the LineageOS for microG project (which are all of my Pixel builds).
Here are details on how to do that from a user that documented the process better than I did
