Hi pacerier,
The former word (Zaiho) means only real treasure like gold plates, jewels and so on. The latter word (Takara) means not just only treasure but also something or someone precious. For example, 'Kanojo wa kare no TAKARA da' means She is his precious one. So you can change the word for another in tne meaning of just real treasure but cannot do in the meaning of precious one.