You’ve been using the WPML setting that adds a query parameter at the end of a URL to select the language
and now you want to switch to the setting where the languages are in directories?
From this setting: Language name added as a parameter (https://www.example.com?lang=fr – French)
To this setting: Different languages in directories ((https://www.example.com/ – English, https://www.example.com/fr/ – French))
Then the below redirect rule is for you. It safely redirects all old URLs to the new ones, after you did the switch.