The farmland was transformed become residential areas.

I have written elsewhere about how Indonesian IELTS candidates often use become to talk about a constant, where in English it is only ever used to describe a change. However, although become is used to describe a change, we would not use become AND transform together.

Actually transform is a good word to use here. Essentially, when a transformation takes place, elements that existed before the change can be recognised after the change, but there is usually a change of appearance and function. However, this is usually expressed without using the word become.

Transform without become

1. New payments technologies are rapidly transforming our lives.
2. Solar photovoltaic systems transform solar energy into electric power.
3. Some early punk bands transformed into hardcore acts.
4. Conversation is soon transformed into preliminary negotiation.

Notice that in examples 2-4 we see both the before and after conditions connected using into, whereas in example 1 there is no into and no mention of the before and after conditions.

Become without transform

If you mention the before and after conditions then you can use become instead of, but NOT as well as transform.

• Solar energy becomes electric power with the use of photovoltaic systems.
• Some early punk bands became hardcore acts.
• Conversation soon becomes preliminary negotiation.

Conclusion

In our opening example the change being described is quite extreme and so become would be rather weak. Better in this case to use transform (but without become!).

• The farmland was transformed into residential areas.