I visited a local SA today, and confirmed the details of what's possible.
You need your order to be in a state prior to completing the financing section. If you already have financing set up, have an SA remove it. That will cause the "edit order" option to appear from the website.
Edit to taste - you are picking from the options as they existed on the day you ordered (and at that day's price, so yes you could get FSD for cheaper).
Once done editing, it's strongly suggested to go ahead and at least click "accept" on a cash-only financing option so the EDD system doesn't toss you to the rear of the queue - it does that with non-finalized orders. You want yours to appear to remain in the ready-to-go state. You can always re-apply for financing as you get towards delivery.
Final nugget - it looks like Tesla financing is likely to go up on April 1st. Lock on March 31 if that matters to ya.