For years and years, Google has been telling people to not change URLs unless they have to. Google's John Mueller said it again in a Reddit thread, strongly encouraging the webmaster to avoid changing the URLs on his site.
In this example, the webmaster is looking to change his international URL structure to /en-gb from /uk and to /de-de from /de, and so on. Google's John Mueller the biggest concern he sees with how Google will react is the URL change and the time it takes Google to reprocess it all. He wrote:
The bigger effect will be from changing a lot of URLs (all pages in those folders) - that always takes time to be reprocessed. I'd avoid changing URLs unless you have a really good reason to do so, and you're sure that they'll remain like that in the long run.
This does not apply with HTTP to HTTPS, because Google has a special process for this.
But for most other URL changes, espesially not at the root level, it can take Google some time to rediscover all those URLs.
Forum discussion at Reddit.
