Changing website’s default language and SEO impact

we have a web site which is example.com (no directory for default language)

Currently the main language is Spanish and we want to change the main language to English instead of Spanish and it shouldn’t affect our SEO.

For example,
“example.com/en/about-en”
should be redirected to the new link directly after changing main language =
(Without EN)
“example.com/about-en”

and “example.com/about-es”, should be redirected to new link = example.com/es/about-es (With ES).

Obviously we don’t want to include “en’ for the default language after changing default language to English.

is it possible to change the main language without losing SEO ranks or facing 404? How the 301 redirects should be written? Thanks