you can change to all 3 options, cash , tesla loan, external loan, once you get VIN. Based on my convo with SA, it is pretty straightforward.
Dont pay too much attention to the calculator.
Your terms, ARP and loan amount is decided when the loan application is filled. e.g you could ask for 50k at 2.49% , but they can only approve 30k at 2.79%. The process is pretty quick and the tesla loan through well fargo or bank o america, is approved within 24 to 48 hours. Again the terms are based on your credit history and all that and not on the calculator.
You can also get a loan approved externally and just provide the info to tesla.
Jut be mindful that for tesla loan (and likely for external) you have 30 days to accept and 60 days for which the loan is valid. After that you have to apply again.