Every 30 days Tesla will send you an email or text asking you to update the mileage of your trade-in. They reevaluate their offer. That happens relatively quickly-- an hour or so. I was worried that they'd lower their offer. Instead, the offer has increased multiple times. I'll update this after I take delivery later this week.
Something to keep in mind: In addition to the sales tax savings you get when trading in, we are now in uncharted territory. If you happen to be selling your car for more than you paid for it, you owe federal capital gains tax. (In the US) If you owned the car for under a year, that short term capital gains rate matches your income tax rate (between 10% and 37%). For example, if you fall in a 20% tax bracket and you are selling for $5k over what you paid, you will owe an additional $1k at tax time. For me, coupling the sales tax savings and the reduced capital gains from Tesla's lower offer, the Tesla trade-in offer is not nearly as far from the vroom/carmax offers as it seemed. Also, if you owned the car for over a year, you do still owe capital gains tax-- but at the long-term rate that is likely 15% for the vast majority of Tesla purchasers, but would be 0% for those earning under $40k or 20% for those earning over $445k.