US Patent 9449470 Wagering game with upgradable symbol stacks

A gaming system includes one or more input devices, display devices, processors, and memory devices. At least one of the memory devices store instructions that cause the system to receive a wager and to display at least one array having a plurality of columns. An outcome is randomly selected, in response to the wager, in which a first column is stacked with a stack of symbols of a first rank and a second column is stacked with a stack of symbols of a second rank. The first rank is compared with the second rank to determine a high rank and a low rank according to a predetermined pay table. The stack of symbols of the low rank is upgraded to a stack of symbols of the high rank.