I had a brief discussion with a group of SEOs about the response John Mueller gave to a webmaster during a Google webmaster hangout back in December. Barry Swartz did a write up on it on Search Engine Roundtable.
The person asked whether changing URL structure during HTTPS to HTTPS migration would be a good idea. To which John Mueller said that changing both HTTP to HTTPS and URLs could actually throw Google off.
With protocol or domain name change where everything else stays the same, Google is able to quickly understand the change and reassign all the signals from the original page to the new page. But when with protocol or domain name, you also change your URL structure, it makes it harder to connect the dots as more things change.
One person from the group (can’t remember who it was. Please let me know if it was you:)) had a good question whether Google has a good understanding of extensions (i.e. .html), and dropping them during migrations would be the same as changing the URL structure.
No one seemed to know the answer, so I decided to ask John Mueller:
.@JohnMu in other words, can dropping extensions throw Google off? 2/2
— Sergey Alakov (@sergey_alakov) May 12, 2017
And here is what John Mueller said:
@sergey_alakov It's a different URL, they're not the same.
— John ☆.o(≧▽≦)o.☆ (@JohnMu) May 12, 2017
While he didn’t say directly that dropping extensions can throw google off, but rather just pointed out that these are different URLs, that is how I understand his answer.
From the SEO perspective, any website migration always presents a challenge to keep site’s organic traffic and rankings at the same level. So, to see Google commenting on nuances like that is always interesting.