How does Google really transfer signals during a domain migration?

What does 'one-to-one copy' really mean for Google?

The concept of one-to-one copy is stricter than one might think. Google compares the URL structure, content, meta tags, and overall architecture between the old and new domains. If the engine detects a perfect match — even tree structure, same content page by page, same metadata — it considers that there is no significant editorial change.

Specifically, this means that every URL from the old domain must redirect to an equivalent page on the new domain with identical or nearly identical content. Minor variations (graphic redesigns, CSS adjustments) are tolerated. However, an editorial overhaul, a merging of pages, or a restructuring of URLs would break this automatic recognition.

Why can Google transfer signals without having to re-crawl everything?

The engine has already analyzed and indexed the old domain. If the new site is an exact replica, Google reuses the data already cached: PageRank, quality signals, structured data, internal links, thematic authority. In other words, the engine does not start from scratch.

This mechanism relies on algorithmic trust: Google detects 301 redirects, verifies the one-to-one correspondence, and then transfers the existing signals to the new domain. Crawling temporarily increases to validate this correspondence, but once confirmed, the engine does not need to re-analyze everything in depth.

What happens to the old domain after the migration?

The old domain does not instantly disappear from the index. Google continues to crawl it sporadically to detect any potential changes (removed redirects, the site going back online). This residual crawl gradually decreases until it becomes negligible.

Signals associated with the old domain — backlinks, history, authority — are transferred to the new one, but this process is not instantaneous. A gradual transition over several weeks is typically observed, and it may take a few months for very extensive or complex sites.

• Strict one-to-one copy: same structure, same content, no editorial restructuring
• Direct transfer of signals: PageRank, authority, structured data reused without exhaustive re-crawl
• Temporary increase in crawl: Google checks the match, then normalizes the activity
• Gradual fading: the old domain remains monitored for a few weeks/months before complete disappearance
• No loss of signals if the migration is clean and meets Google's expectations

Does this statement align with real-world observations?

Yes, but with important nuances. In practice, 'perfect' migrations are rare. Even with clean 301 redirects and an identical structure, fluctuations in rankings are often observed for 2 to 6 weeks. Google claims to transfer signals without re-crawling, but the engine still needs to validate the match, which takes time.

Feedback shows that sites with a high volume of pages (>10,000 URLs) or historically limited crawl budgets experience longer transitions. The transfer is not instantaneous, even if Google presents it as automatic. [To be verified]: Does the actual speed of transfer depend on the crawl budget allocated to the new domain?

What are the practical limits of this mechanism?

The condition of 'one-to-one copy' is extremely restrictive. In most migrations, we take the opportunity of changing domains to optimize the structure, merge duplicate content, or restructure categories. Once we deviate from a strict one-to-one, Google can no longer automatically transfer signals.

This leads to a hybrid scenario: some pages benefit from direct transfers, while others require re-crawling and a complete reevaluation. This explains why many migrations yield mixed results despite clean redirects. Martin Splitt's statement describes the ideal case, not the majority of real migrations.

Is it really necessary to maintain redirects indefinitely?

Google does not provide an official minimum duration, but experience shows that a minimum of one year is prudent. Some historical backlinks remain active for years. Removing redirects too early — say, after 6 months — can lead to partial loss of signals, especially if external links still point to the old domain.

On the other hand, maintaining redirects beyond 2-3 years becomes unnecessary if crawling of the old domain has dropped to zero and no active links remain. [To be verified]: Does Google continue to transfer signals after several years of redirects, or does it consider the transfer definitively finalized?

What concrete steps should be taken to ensure signal transfer?

Before any migration, map out completely the structure of the old site. Each URL must have an exact match on the new domain. No orphaned URLs, no redirects to the homepage by default, no chains of redirects. A clean mapping is the sine qua non condition for automatic transfer.

Then, check that the content is strictly identical: same title, same meta description tag, same Hn structure, same main text. If you take advantage of the migration to optimize content, accept that Google will need to reevaluate these pages, which will prolong the transition. The choice between a pure migration and editorial overhaul must be made from the start.

What mistakes should absolutely be avoided during a migration?

The first mistake is to change multiple variables simultaneously. Domain migration + graphic redesign + URL restructuring + content optimization = explosive cocktail. Google can no longer identify what relates to technical transfer and what constitutes an editorial change. The result: complete re-crawl, unstable positions, temporary loss of visibility.

Another classic pitfall: launching the migration without monitoring the crawl of the old domain. If Google continues to crawl the old site heavily several weeks after the migration, it is often a sign that redirects are not being detected properly, or that the new site presents inconsistencies (different content, 404 errors, blocking robots.txt).

How to check that the transfer is happening correctly?

Use Search Console on both domains simultaneously. Check that impressions and clicks from the old domain gradually decrease while those from the new increase. A successful transfer demonstrates a mirrored curve: regular decline of the old, regular increase of the new, with a clear crossover after a few weeks.

Also monitor the coverage reports: URLs from the old domain should go to 'Redirected' and then disappear from the index, while the new domain should show a growing number of indexed URLs. If the new domain stagnates or shows a significant error rate, dig deeper immediately: crawl issue, blocked content, inconsistencies detected by Google.

These technical optimizations require a high level of expertise and constant vigilance throughout the transition phase. If you are not accustomed to managing this type of project or if your site has complex specifics (multilingual, numerous facets, heavy JS architecture), it may be wise to seek assistance from an SEO agency specializing in migrations. Personalized support helps to anticipate pitfalls, react quickly to anomalies, and secure the signal transfer.

• Establish a comprehensive and rigorous URL mapping before going live
• Ensure a strict one-to-one match between the old and new domains (content, structure, metadata)
• Set up clean 301 redirects, without chains or loops
• Activate Search Console on both domains and declare the address change
• Monitor the crawl of the old domain and check for its gradual decrease
• Monitor impressions/clicks: mirrored curve = successful transfer