I hadn't touched my order in a couple months, but decided to make a change last night to the white interior and exterior color and the backdoor edit link was working. There was a another backdoor link floating around that was working for a while, but it seemed to stop working in the past couple weeks. All the pricing was the same as when I placed my original order at the end of June except for the destination charge, but once my account page updated even that was showing the original destination charge. So, I think you can make whatever edits you like and you'll get the pricing you would have gotten on your original order. Well, at least until you get a VIN, slay a dragon, or some other mystical rite deems you worthy of delivery.
The following link however is still working and I just checked it again:
h t t p s://www[dot]tesla[dot]com/teslaaccount/edit-design/RNXXXXXXXXX