I noticed today that when I go into "start designing" I am no longer able to and get the below page. Could they be getting ready for an update after the earnings report? I noticed this on my desktop using Firefox and IE.
No. That's not usually how pages are updated/replaced. They develop and test the new pages/functionality behind the scenes and then swap it out all at once. They don't take down perfectly good pages to put up some intermediate, non-functional page, as a place holder. They haven't done it with any other updates so I don't expect they'll start now.
They often do A/B testing on new pages. Not everyone will see the new stuff for a while. For 2 months, one of my household computers saw one website (new design) while others saw another. now all of them see the new one.
Yes they have, many times. There are a few egregious instances where a test page was loaded on the production site before they were intended to be loaded or released, but then quickly (within a few hours) taken back down. That would never happen with a test->staging->production type process.
I don't remember when, but Tesla also leaked/released a new Model S/X design studio page to be much like the Model 3 page. It was clearly unfinished and put up for beta testing. I'm sure there's a thread around here somewhere about it. If they wanted to beta test an all-new design studio page, they surely didn't need to do it in production visible to the entire world.
They also do staggered releases based on some attribute like with the CPO/Inventory pages. For quite a while, both the old and new pages were displayed to different users.