Units expire after 24 months. Do not hoard them; spend them on the next download/update purchase.
Downloading an update does delete or block earned Uplay/Ubisoft Connect achievements, but syncing issues can occur. These are usually fixed by clearing the cache or re-triggering an achievement online. uplay ach earn achievement download upd
(loyalty points) primarily by leveling up your account through Challenges or by making purchases in the Ubisoft Store. Units expire after 24 months