That
It would be really easy to implement a simple system where you can choose to either adopt the name of your new civ or keep the old one, it wouldn't change any mechanics and it would go a long way towards making everyone happy, both those who like the idea of changing their civs and those who dislike the idea that the civ they choose to play at the start won't "stand the test of time"
Hopefully this is something they add at the same time they add custom religions (those were not in the exploration age stream but mostly confirmed to be added before release)