What does upgrading an expression do?

Solution 1:

Generally the 2nd level of an expression adds in some sort of lighting effect. This doesn't effect how the game works, but rather adds another level to how you can communicate with other players.

The game still allows you access to the 1st level of the expression (1st level is used when you tap the expression.) For this reason you'll have to hold the expression button slightly longer to activate the 2nd or higher level of an expression. You'll be able to tell what level you're using based on which dot above the expression is lit up when you release the expression button. These dots are not present until after that expression has more than 1 level.