"We are going to change."
Translation:Nós vamos mudar.
I'm not sure how does it sound to a native speaker, but it is wrong for phrasal future. There is a difference between Portuguese and Spanish here:
Both languages have a construction similar to the English "going-to" future. Spanish includes the preposition a between the conjugated form of ir "to go" and the infinitive: Vamos a cantar 'We're going to sing' (present tense of ir + a + infinitive). Usually, in Portuguese, there is no preposition between the helping verb and the main verb: Vamos cantar (present tense of ir + infinitive). This also applies when the verb is in other tenses:
- Ayer yo iba a leer el libro, pero no tuve la oportunidad. (Spanish)
- Ontem eu ia ler o livro, mas não tive oportunidade. (Portuguese)
- Yesterday I was going to read the book, but never had the chance.
Is it that that would need an actual object (which can be implied in English)?
Yes; trocar is (according to Wiktionary and other references) always transitive or ditransitive with com or por (“ditransitive” being something like English’s “give the ball to the dog”, a verb with a direct and indirect object, which may be within a prepositional phrase). Intransitivity doesn’t make sense.
And trocar’s uses don’t correspond to “We will change”—it would sound like “? We will substitute”, “? We will swap”, which sound wrong unless the object were not just implied, but immediately implied, (as in, “Q: This one’s too big for me!” “A: Then we will swap.”).