According to some implementations, when a triggering symbol lands in a defined area of a slot game display during an instance of a base game, there is a chance that the triggering symbol may change to a new symbol. If so, the new symbol may be held in the defined area and may persist during one or more additional purchased base game instances. The new symbol may persist if at least one additional triggering symbol lands in the defined area in subsequent bought games. Each additional triggering symbol may automatically change to a new symbol as the additional triggering symbol lands in the defined area of the slot game display. Whether the triggering symbol will change to a new symbol may involve a random component. In some examples, the greater the number of triggering symbols, the greater the probability the triggering symbol will change to the new symbol.