I am confused about using transitive and intransitive verbs for making passive sentences. Especially when that verb can be both (like the verb change).
Change is one of a number of ergative verbs in English. These are used in both transitive and intransitive senses:
In the transitive sense, the Agent is the subject of the verb and acts upon a Patient, the direct object.
John (Agent, subject) broke the window (Patient, direct object).
Mary (Agent, subject) walked the dog (Patient, direct object).
This book (Agent, subject) changed my life (Patient, direct object)
In the intransitive sense, the Agent is deleted and the Patient becomes the subject of the verb, which thus represents the action as “performed” by the Patient.
The window (Patient, subject) broke.
The dog (Patient, subject) walked.
My life (Patient, subject) changed.
As you know, only transitive verbs can be cast into the passive, but the effect is very similar — the Patient becomes the subject. In a passive construction, however, the Agent may be restored as the object of a prepositional phrase with by:
The window (Patient, subject) was broken by John (Agent).
The dog (Patient, subject) was walked by Mary (Agent).
My life (Patient, subject) was changed by this book (Agent).
Ergative verbs mostly designate a change of state, and there is often very little difference, if any, in meaning between the transitive and intransitive versions. The same outcome is described, and the difference is largely one of focus — the transitive sense puts the emphasis on the Agent, the intransitive sense puts the emphasis on the outcome and ignores the Agent altogether.
Tom boiled water. The water boiled.
This book changed my life. My life changed.
However, there is substantially more difference when the Patient is animate and the verb designates a change which the Patient is capable of effecting by its own effort:
Mary walked the dog through the park. The dog walked through the park.
Lee moved Stuart to the left wing. Stuart moved to the left wing.
Verbs can form passives only when they are used transitively. The active sentence The government changed the law is transitive, and so allows the passive construction The law has been changed by the government. On the other hand, You have changed a lot over the past ten years is intransitive when it means that the person addressed has a different appearance, and so does not allow a passive form.