You do realize that once you are no longer President anymore that both the titles President Donald J Trump and former President Donald J Trump are both completely accurate and accepted, right?

Exactly the way that if you were talking to them to their face you would still call them President Obama, President Carter, President Clinton, but if you're speaking in reference to them you can either call them President Obama, President Carter, President Clinton or former President Obama, former President Carter, or former President Clinton

Both ways are correct and always have been correct for a former President......