If they do it like S/X, you won't lose your place in line. Tesla gated access to the config tool based on deposit date. Only when you placed an order did you reserve your true place in line. Tesla will continue to grant access to the config tool in lots. If you don't order, the people in the lots after you can place orders, thereby getting in front of you.
If you wait for features (D, P, whatever), you'll likely be able to see the config tool with those features as soon as they are available. Anyone who has been granted access to the config tool up until that point will be able to place an order, so it will behoove you to order quickly.
Again, this is if tesla follows the same process as S/X. It's entirely possible they will come up with a different method, because by the time the P and D are made available there could be 50k people with access to the tool. That would be fun... we could watch the estimated delivery times change by the hour.