En otras palabras, el plan ha cambiado.

En otras palabras is a fixed expression meaning in other words. It is used to rephrase, clarify, or summarize what was said before.

So in this sentence, the speaker is introducing a simpler or clearer restatement of an earlier idea.

It works very much like English in other words:

• Es demasiado caro. En otras palabras, no lo voy a comprar.
• Está ocupado; en otras palabras, no puede venir.

Because it is such a common set phrase, it is best learned as one whole chunk.

Because palabras is a feminine plural noun.

In Spanish, adjectives must agree with the noun they describe in gender and number:

• singular masculine: otro
• singular feminine: otra
• plural masculine: otros
• plural feminine: otras

Since palabras is feminine plural, the adjective must also be feminine plural: otras.

Because the expression is fixed as en otras palabras, literally in other words.

English also uses the plural here: other words, not other word. Spanish does the same. It refers to saying the same idea again using different wording.

So en otra palabra would sound wrong in this context.

Because en otras palabras is an idiomatic expression, and Spanish normally does not use an article there.

En las otras palabras would sound unnatural for this meaning. It would suggest something much more literal, like referring to some specific words already mentioned, not the expression in other words.

So for the meaning of reformulating what you said, you should learn:

• en otras palabras

not:

• en las otras palabras

Because En otras palabras is an introductory phrase. In Spanish, these expressions are usually separated from the main clause with a comma.

So:

• En otras palabras, el plan ha cambiado.

This is similar to English punctuation in:

• In other words, the plan has changed.

You will often see commas after connectors and discourse markers such as:

• Sin embargo, ...
• Además, ...
• Por ejemplo, ...
• En otras palabras, ...

Plan is a masculine singular noun, so it takes the masculine singular definite article el.

Spanish nouns normally need an article more often than English nouns do. So where English says the plan, Spanish says el plan.

A few examples:

• el plan
• el problema
• el cambio

Here, el plan means the plan, referring to a specific plan already known from the context.

Because Spanish often leaves out subject pronouns when they are not needed.

The verb form ha cambiado already tells you the subject is he/it/you formal has changed, and here the noun el plan is stated explicitly, so adding a pronoun would be unnecessary.

Spanish prefers:

• El plan ha cambiado.

not:

• El plan él ha cambiado.

That would be incorrect.

More generally, Spanish is a pro-drop language, meaning subject pronouns are often omitted:

• He llegado = I have arrived
• Han salido = They have left

Ha cambiado is the pretérito perfecto compuesto, often called the present perfect.

It is formed with:

• haber in the present tense + past participle

So here:

• ha = third-person singular of haber
• cambiado = past participle of cambiar

Together:

• ha cambiado = has changed

In Spain, this tense is very commonly used for actions or changes that are connected to the present, especially when the result matters now.

Both can often translate as changed, but they are not used in exactly the same way.

In Spain, ha cambiado is very natural when the change is seen as relevant to the present situation. It suggests something like:

• the plan is now different
• the current state matters now

Cambió is the simple past (pretérito indefinido), which usually presents the change more as a completed event in the past.

Very roughly:

• El plan ha cambiado = The plan has changed
• El plan cambió = The plan changed

In Peninsular Spanish, the present perfect is especially common with time expressions connected to the current period, such as:

• hoy
• esta semana
• este mes

So this sentence sounds very natural in Spain.

Cambiado is the past participle of cambiar.

For regular -ar verbs, the past participle ends in -ado:

• hablar → hablado
• trabajar → trabajado
• cambiar → cambiado

For comparison:

• regular -er / -ir verbs usually end in -ido
• comer → comido
• vivir → vivido

In this sentence, cambiado is used with haber to form the present perfect:

• ha cambiado

It can be used on its own here.

Cambiar can be:

• transitive: something changes something
• intransitive: something changes

Examples:

• Han cambiado el plan. = They have changed the plan.
  Here it has an object: el plan.
• El plan ha cambiado. = The plan has changed.
  Here el plan is the thing that changes.

So in your sentence, el plan is the subject, not the object.

Grammatically, you could place en otras palabras elsewhere, but it would usually sound less natural in this exact sentence.

The most natural position is at the beginning when it introduces a reformulation:

• En otras palabras, el plan ha cambiado.

If you put it at the end:

• El plan ha cambiado, en otras palabras.

it sounds marked or stylistically unusual, because the phrase is normally used to introduce the restatement, not to add it afterward.

So for everyday Spanish, the beginning position is the best choice.