I don't think he's switched allegiance, but it's more of an administrative mix up. James played ATP Challengers - his first pro events from 2020 to October 2023 - as British. In the ITF event a week after his last ATP Challenger in 2023 and the ITFs since finishing college in May he's appeared as USA. The ATP never changed his nationality from GBR during that time, so I'm inclined to think that it's just a mistake that he showed as USA on the ITF site.