I remember I suggested that once Tier 100 was reached for the Enchanted Springs, there was a very low chance that the option of 'freezing' your creatu could pop up. That way the creatu would cease to age, even if it was on your profile.
So I must say, I support the idea of having high-level rewards.
The species changing might work. However it might also interfere with nametags, because instead of tagging a new name to a specific pet, you could always try to change the original pet to the species you wanted the name on.