grammar - present perfect tense usage

To understand the answer to your question you need to know that change is an ergative verb. Wikipedia defines ergative verbs as follows:

In linguistics, an ergative verb is a verb that can be either
transitive or intransitive, and whose subject when intransitive
corresponds to its direct object when transitive.

So, in the sentence I have changed my phone number, change is a transitive verb with I as the subject and phone number is the object.

But change can also be used intransitively, as in My phone number has changed. We can see how, in Wikipedia's definition, the "subject when intransitive corresponds to its direct object when transitive".

Another common ergative verb is break:

I broke my watch (object).

My watch (subject) broke.

Wikipedia lists the following as common ergative verbs:

Ergative verbs can be divided into several categories:

• Verbs suggesting a change of state — break, burst, form, heal, melt, tear, transform




• Verbs of cooking — bake, boil, cook, fry




• Verbs of movement — move, shake, sweep, turn, walk



• Verbs involving vehicles — drive, fly, reverse, run, sail

You can also say Everything has been changed, in which case you are using the transitive change in the passive. The difference between Everything has changed and Everything has been changed is that the latter implies that there is an agent behind the change, in a way that the former does not necessarily.