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