We did precisely the same thing at our company about 18 months ago. Our original codebase just couldn't scale. It was a great MVP platform, and we learned a great deal from it, but it simply couldn't handle load, sophisticated billing options, or any reasonable expansion of the product functionality.

That said, we left the new and old systems running in parallel (all new users were directed to the new platform) for over a year before we even informed users we were shutting it off, then gave them another six months to migrate. And we never purged the old data; it's in cold storage in case someone ever comes looking for it.

Rewriting the codebase isn't something you should take lightly, but "never" is pretty damn strong word.

Then it seems like you're not really rewriting as Joel defines it, you're launching a new product (it does new things, and it doesn't replace the old one). I guess it then becomes a question of effort spent in cannibalizing your own product vs. the income the new version brings, which is a very different equation for Basecamp and the products Joel talks about.