I plan to change it: set the main urls as French, and create /en/...
paths for the English ones.
If I correctly understand you, your main language is currently English.
In this case I would highly recommend yout not to change the main language. If you plan to have the website content in two languages, I would recommend, you let the current website structure and the language in the current form. The only thing you'll do: add a folder /fr/ for the french content.
Further, your english and french content versions become their hreflang links. And, if you mean, your certain language version should be counted by Google as main, you markup this version with hreflang x-default.
This solution prevents the case, where your english content is moved anywhere - therefore you avoid its full re-indexing. The only new things Google will get into account, are the new /fr/ content folder, and new hreflang attributes.