A symbol upgrade turns lower-paying symbols into higher-paying ones. It may happen to symbols in view on a single spin, or it may be a lasting upgrade where, for example, every copy of the lowest symbol becomes the next one up for the rest of a feature.
Progressive upgrades are common in free spins: collecting a set number of a special symbol removes the lowest low-paying symbol and replaces it with a high-paying symbol, so the reels become richer as the round goes on.
Upgrades shift the value of later spins without changing the stake, which is why features with them can pay well when they run long. Their value is already in the game's RTP. See our symbol upgrades feature page.