GA4 Attribution Settings: Changing the Model Rewrites Past Data, Lookback and Channel Changes Do Not

A client's year-over-year conversion report changes overnight, and nobody touched the campaigns. Last March now shows fewer paid conversions than the report you sent last March. Usually someone changed one of the GA4 attribution settings, and the confusing part is that GA4 treats those settings in opposite ways: one rewrites every past date, the others only count from the day you save.

This matters to any agency that reports GA4 key events next to Google Ads results, or compares this year with last. Below is what each setting does to old data, what changed in 2026, and how to keep a client report honest when a setting moves.

Where are GA4 attribution settings?

GA4 attribution settings live in Admin, under Data display, then Events, then Attribution settings, and you need the Marketer role or above on the property to change them. Google's help page on selecting attribution settings lists three controls on that screen: the reporting attribution model, the channels that can receive credit, and the key event lookback window.

Write down all three for every client property at onboarding. If a number moves later, the first question is whether one of them changed, and you cannot answer that without a starting point.

Which GA4 attribution setting rewrites historical data?

The reporting attribution model is the GA4 attribution setting that rewrites historical data: Google says a model change "applies to historical and future data." Switch a property from data-driven to a last click model in October and every past month is recalculated under the new model, including the months you already reported.

Google is specific about what moves. The change shows up in key event reports and explorations that use event-scoped traffic dimensions such as Source, Medium, Campaign and Default channel group, and it affects key events, total revenue, purchase revenue and total ad revenue. User-scoped and session-scoped traffic dimensions, such as Session source or First user medium, are unaffected.

Two smaller notes from the same page. If a date range starts before a model's start date, you see partial data. And the first click, linear, time decay and position-based models were removed in November 2023, so very old reports built on them cannot be reproduced at all.

Which settings only apply going forward?

The key event lookback window and the channels that can receive credit both apply going forward only. Google says lookback window changes "apply going forward" and show in all reports in the property, and that a change to channels that can receive credit also applies going forward and to every linked Google Ads account.

The lookback options are fixed. Acquisition key events (first_open and first_visit) default to 30 days, with 7 days as the alternative. All other key events default to 90 days, with 30 or 60 as the alternatives. The channel setting has two choices: Paid and organic channels, or Google paid channels. Google notes that Paid and organic channels only applies to web conversions; app conversions always use Google paid channels.

Because past data is not recalculated, a lookback change splits the property's history in two. Shorten the window from 90 to 30 days on 1 July and conversions from July onward are counted under a stricter rule than conversions from June, with no visible marker in the report.

What changed with GA4 conversion windows in August?

Since 11 August 2026, GA4 conversion windows accept any whole number of days: 1 to 90 for click-through conversions and 1 to 30 for engaged-view conversions. Google's Analytics release notes say engaged-view windows were previously fixed at 3 days, and click-through windows were limited to presets of 1, 7, 14, 30, 60 or 90 days. You set them in Advertising, then Conversion management, or in the linked Google Ads conversion management screen.

The release note does not say whether a window change reaches back. Google Ads help on conversion windows does: changes apply to conversions going forward and are not counted retroactively. The same page gives the defaults (30 days click-through, 3 days engaged-view, 1 day view-through), recommends windows of at least 7 days, and explains who owns the setting. With Google paid channels you change windows in Google Ads; with Paid and organic channels you change them in Google Analytics.

That last point catches agencies out. If GA4 is set to Paid and organic channels, a PPC specialist editing windows in Google Ads may be looking in the wrong place.

What is per-conversion attribution in GA4?

Per-conversion attribution is a GA4 beta, announced on 16 January 2026, that lets attribution settings be adjusted independently for every web conversion instead of one setting for the whole property. Google says it is meant to fine-tune Google Ads bidding and reduce reporting gaps with Google Ads, and that it may not be available to every property. A 29 September 2026 release note extended the same control to app conversions.

For reporting, this means two conversions in the same property can now follow different rules. A lead form and a purchase can sit side by side in a client report on different windows, so label each one.

How do GA4 attribution settings break year-over-year reports?

GA4 attribution settings break year-over-year reports in two opposite ways: a model change silently rewrites last year, while a window change silently mixes two rules inside one comparison. In the first case the PDF you sent a year ago no longer matches GA4. In the second, GA4 is internally consistent but the trend is not like for like.

A quick test tells you which one you are looking at. If last year's numbers in GA4 differ from the report you sent last year, suspect the model. If last year still matches but the trend bends on a specific date, suspect a lookback or conversion window change on that date.

How should agencies report around attribution changes?

Agencies should report around attribution changes by logging every setting change with its date, anchoring year-over-year on dimensions the model does not touch, and telling the client before the numbers move. A practical routine:

1. Record a baseline. Note the model, channel setting, lookback windows and any per-conversion windows for each client property, with the date.
2. Keep a change log. Google's attribution settings page does not describe a history for these three settings, so keep your own: who changed what, when and why.
3. Get sign-off before a model change. Explain that it will restate past months, and save a copy of the last report under the old model.
4. Use session-scoped dimensions for long trends. Key events by session default channel group are unaffected by the model, so they hold steady across a switch.
5. Annotate window changes. Put the change date on every chart that crosses it, and compare periods on the same window where you can.

For layout, our guide to GA4 dashboards for agencies covers what belongs in GA4 versus the client report, and the PPC report template shows where conversion notes fit. A Google Ads reporting tool that puts Google Ads and GA4 in one branded report makes the attribution note easy to keep next to the numbers it explains.

GA4 attribution settings FAQ

Does changing the GA4 attribution model change historical data?

Yes. Google says changing the reporting attribution model applies to historical and future data. Key event reports and explorations that use event-scoped traffic dimensions, such as Source, Medium and Campaign, are recalculated for past dates too.

Does changing the GA4 lookback window change past data?

No. Google says changes to the lookback window apply going forward. Past periods keep the window that was in place at the time, so a before and after comparison mixes two rules unless you note the change date.

What lookback windows can I choose in GA4?

For acquisition key events (first_open and first_visit) the default is 30 days, with 7 days as the alternative. For all other key events the default is 90 days, with 30 or 60 days as the alternatives.

Can I set custom conversion windows in GA4?

Yes. A GA4 release note dated 11 August 2026 lets you set click-through conversion windows to any whole number from 1 to 90 days and engaged-view windows from 1 to 30 days, which were previously fixed at 3. They are set in Advertising, Conversion management.

Who can change GA4 attribution settings?

Google says you must be a Marketer or above at the property level. The settings are in Admin, under Data display, then Events, then Attribution settings.