The Forsaken Annual pass was not able to be purchased with silver, and functioned differently than the current Season Pass model. When Shadowkeep first launched, and the new Season Pass format was introduced, was when seasons were made an in-game purchase (buyable with silver, or via expansion bundles only). Bungie did originally offer that excess seasons purchased, such as buying a single season, and then upgrading to Deluxe, would provide silver for the extra seasons. That offer had to quickly be stopped, due to exploitation and rampant abuse by players. As such, any excess seasons are simply lost. Seasons also cannot carry over outside of the content year, so excess year four seasons cannot unlock seasons for year five.