Also, I understand your confusion in 301 vs 302. I mean essentially, your redirect is temporary. But 302 redirects are meant more so for short time periods, probably with a month being the max amount of time you would want to do this. You'll want to use a 301 to maintain linking power. There's no harm in doing so and then replacing it later.

You would remove the old 301 redirect from www.example.com/fr/ to www.example.com and add a new one from www.example.com/fr/ to www.example2.com/fr/ , then force a recrawl on your site.