You could just look this up. They renamed to Basecamp because they decided to be a single-product company (at the same time, they divested Highrise and Campfire). Six years later, they launched HEY, their email product, so "Basecamp" stopped making sense as a name. They wrote a post about this last year.

later

I added "six years later", but I don't think it changes the meaning of what I wrote originally.

Most HN people probably never stopped calling them 37signals, so this seems like an especially weird thing to get hung up on.

Are they dead-naming a company? Isn't that illegal in California already?

That decision was always baffling to me. Basecamp is such a UX nightmare even Jira looks good next to it...

I don’t think they were ever Basecamp Inc, I think they were always an LLC.