If someone registered a popular brand's username early on, can the brand claim it later?

Let's say someone created an account with the username of a product or brand before it became popular (like ChatGPT or Twitter). Does that person get to keep the username indefinitely, or can the actual brand/company claim it somehow?

I'm curious how this works, is it first come, first served forever or do companies have a way to reclaim usernames that match their trademarks?

Has anyone experienced this or know Digg policy on this?