:We use the nationality held at the time the subject became notable in the lead sentence. Thus the lead sentence should read "American", as should nationality in the infobox. This is precisely why there is also a citizenship field in the infobox, for changes of citizenship, so that field should read "British". This has all been done and is a good solution. We specifically don't use hyphenated or phrases like "American-born British" in the lead sentence, see WP:OPENPARA.