Basecamp could definitely use a refresh, but as a daily user I do worry something will be lost if it's completely redone.
2. Nothing changes with the Basecamp you already know and love. The new Basecamp is an entirely different product. You can use whichever you prefer, no one is being forced to change or switch.
Good on you. This is how it should be done.
Unless you're building the whole new product that serves a different purpose, and is only influenced by Basecamp - in that case, it makes sense.
Basecamp has been awesome for a wide range of users, but we're seeing other folks taking a stab at this space (e.g. https://trello.com/) and eventually someone is going to come up with something that is all around better for the average user. Why wait for the inevitable?
It doesn't do some of the things the existing Basecamp does.
It does new things the existing Basecamp doesn't do.
It's a new product with new ideas about what's important in a product like Basecamp. It's not the same as today's Basecamp, but it's still familiar to the millions who've used Basecamp.