Faturalandırma bu ay değişti.

Faturalandırma means billing or invoicing. It usually refers to the process or system of billing, not to a single bill.

• fatura = invoice, bill
• faturalandırma = billing, invoicing

So this sentence is about billing as a process, not about one individual invoice.

No. Here, faturalandırma is a noun.

It comes from the verb faturalandırmak (to bill / to invoice), but the -ma here turns it into a noun: billing / invoicing.

This -ma is not the negative suffix in this sentence.
Compare:

• faturalandırma = billing / invoicing
• değişmedi = it did not change

So in your sentence, the actual conjugated verb is değişti.

Yes. Faturalandırma is the subject.

The verb değişmek means to change in the sense of to undergo change. That means the thing that changes is the subject.

So the structure is basically:

• faturalandırma = billing
• bu ay = this month
• değişti = changed

If you wanted to say that someone changed the billing, Turkish would usually use değiştirmek instead:

• Faturalandırmayı değiştirdiler. = They changed the billing.

Because in Turkish, time expressions are often used without a case ending.

So:

• bu ay = this month
• bu yıl = this year
• bugün = today

That is the normal way to say when something happened.

Using bu ayda here would sound unusual or marked in ordinary speech. For this sentence, bu ay is the natural form.

Değişti is in the simple past tense.

It is built from:

• değiş- = change
• -ti = past tense

Depending on context, English may translate this as either:

• changed
• has changed

So Turkish değişti does not always map to just one English tense.

The past tense suffix is often shown as -DI in grammar books, because its form changes.

In değişti:

• the vowel becomes i because of vowel harmony
• the consonant becomes t because the verb stem ends in ş, which is voiceless

So:

• base verb: değiş-
• past suffix: -DI
• actual result: değiş-ti

This is completely regular.

Because Turkish does not have articles like a/an and the.

So faturalandırma can mean:

• billing
• the billing

The exact English choice depends on context. Turkish usually leaves that to the situation rather than marking it with an article.

The sentence has a natural Turkish order, and it is also somewhat flexible.

• Faturalandırma bu ay değişti.
• Bu ay faturalandırma değişti.

Both are possible.

Starting with faturalandırma puts billing first as the topic:
As for billing, it changed this month.

If you start with bu ay, the time expression is a little more prominent:
This month, the billing changed.

The most important fixed element is that the verb değişti normally comes at the end.

Because değişmek and değiştirmek are different verbs.

• değişmek = to change, to become different
• değiştirmek = to change something, to alter something

So:

• Faturalandırma değişti. = The billing changed.
• Faturalandırmayı değiştirdi. = He/she changed the billing.

Your sentence uses the intransitive idea: the billing itself changed.

Not normally.

Faturalandırma bu ay değişti means that the billing changed. It does not explicitly say who caused the change.

If you want was changed, Turkish would usually use a different form, such as:

• Faturalandırma bu ay değiştirildi. = The billing was changed this month.

So değişti and değiştirildi are not the same.

Yes, very often you could.

In context, Faturalandırma bu ay değişti may be expressed in natural English as:

• Billing changed this month
• Billing has changed this month

Turkish -di is a past form, but English sometimes prefers the present perfect when the change is still relevant now. So if your translation uses has changed, that can be perfectly reasonable.