SEO impact on switching the landing page language

That's really interesting question, Google generally consider it as some kind of mistake by webmaster and will lower the position of that page, for example when site is hacked by someone, Google generally re-crawl that page and then if they see other things other than original content, which they usually see from a long time whenever they re-crawl your webpages, then they might doubt on your site if they see sudden changes, and might be rank down your site for a moment, unless webmaster confirm such a changes. I don't know how Google will treat in your case, since it is you(not a hacker) who make such a kind of changes.

You can surely do it right way, and Google will understand that but it will take much longer time, plus it will rank down your position. I don't know how much time it will take to recover your position but you can wait for it if you're fine. Or you can try below two safe option.

  1. Place English language content in your subdirectory, and keep the original content as it is. I don't know what's the problem about that?

  2. Redirect your old webpages(other lang url) to subdirectory and keep that for a long time like 1-2 month, don't worry your position will recover in only 1 week or even in few days, since you changed only URL not a content. After Google recognize your new URL with same content and rank your pages again normally, then you can put your English content in your old URL, and then remove that 301 redirection.