To perform a creature swap, all the player has to do is guide his/her creature toward the new one so that they are close to each other and then click the silver reward scroll on top of the new creature's head. Creature swapping is not permanent and the player can undo the switch if he/she so desires. Creatures obtained via trade will have their size slighly reduced (it is barely noticeable), but all its attributes will be roughly the same, aside from the creature's inherent behavioral tendencies (Cows tends to seek food more often even when not hungry) and the new looks, obviously.