Select "Promotions" at the "Extras" menu, then enter one of the following codes.

Upgraded Gravattac
Enter Feedback, Fourarms, Diamondhead, Bloxx as a code.

Upgraded Wildvine
Enter Gravattack, Eye Guy, Arctiguana, Wildmutt as a code.

Upgraded XLR8
Enter Shocksquatch, Gravattack, Arctiguana, Diamondhead as a code.