となる

This phrase describes that a situation will change into the state mentioned earlier. Since it implies this is the change to the final stage, it's not natural to use it with words that are hard to define as a final stage, such as "にぎやか" (lively), "病気" (sick), "元気" (healthy).

All cases using 「....となる」can be changed to 「...になる」, but not every usage of 「になる」 can be changed to 「となる」.

X　にぎやかとなった。
O　にぎやかになった。 Became lively.