A Feature Upgrade is a very special 'meta' feature that, rather than paying out winnings itself or directly affecting other symbols, enhances another special feature symbol (e.g., Collector, Payer) already on the reels to a more powerful 'Persistent' version.
The core of this feature is to change the 'nature' of other special symbols that determine the potential of the bonus round, instantly transforming what could have been an average bonus into a round where one can expect the Max Win.