Thanks for all your kind answers, I still have a few questions though.
1. Would it be possible to change the actual species of the pet to its evolved form? In pokemon for instance, the species Charmander changes into Charmenleon after evolution.
2. Would it be possible to make more than one evolution stage? Digimon can have up to five evolution stages while pokemon may have two.
I'd appreciate it very much if someone can answer the two questions above.