I don't get the impression that HN is desperate to keep growing and bring in new users. Usually it seems like the opposite is true.

Visiting MetaFilter this morning from this story, my first thought was "They're still using that same look?!"

There are many ways to gradually modernise a site without terrifying the herd (the way Slashdot has frequently in its past) - font and font size, headers and gradually the colour scheme, improve on-boarding, etc. It doesn't have to be done in one hit.