Can “become” and “turn into” be used interchangeably?

Question

Can "become" and "turn into" be used interchangeably?

“To turn into” is to transform.

“To become” is to grow to be.

Because "to turn into" means is to transform, it can be used to mean "transform instantly". However, "to become" seems to imply that the transformation takes time, so it can't be used in the case of a person turning instantly into something through a magic spell.

Is this the case?

For example:

The woman turned into a lion, because the fairy cast a curse on her.

The woman became a lion, because the fairy cast a curse on her.

Answer 1

As with many English synonyms, the connection is based on only one specific definition of each, in a context where they do mean the same thing.

The once-small, street-smart startup has become/turned into a stodgy internet powerhouse.

As you say, to "turn into" means to "transform". In this case, "become" also means the same thing. However they are not interchangeable in a different sentence such as:

She became angry at his stubborn refusal to leave.

where there is a gradually change in feeling, or in something like:

The enchantress turned the handsome but vain prince into a beast.

where the prince was rapidly changed from one thing into another.

To be clear: It's not the length of time involved but rather a difference in focus. "Turn into" focuses on the change from A to B. "Become" focuses on the process of changing from A to B.

Moreover, as Ronald Sole points out, there is the other definition of "turn" to mean "change direction". You can, while driving, "turn into" a driveway. So you have to be careful of these and other exceptions.