Oliver, it is not possible to keep both urls due to the inner design of the addon. Does the rank get lost if the old page is redirected to the new page? If the rank can be re-linked to the new location, then I see no major problem here.
I was working on my version of the addon that keeps the old htaccess rewrite rules for non-lc urls with redirect to default language. I could successfully make that old known pages were redirected to the new lc-prefixed locations.