No. They've put to much time, testing, and investment into their current pack. Changing it now would be a huge risk. They would be essentially putting an untested, un proven technology into a product that has to come out strong at the start.Also could you see Rivian changing their current battery pack to a newer one before official sales begin?
I do think that they will have longer ranges than they currently advertise. In an interview earlier this year RJ mentioned the following numbers:
R1T/R1S 105kWh: 240/250
R1T/R1S 135kWh: 315/330
R1T/R1S 180kWh: "Well over 400"
On that last one, if you assume the same rate of increase from the base to mid you come up with about 420 for the R1T and 445 for the R1S. That would be after you subtract a few miles for the added weight of going from an additional 30kWh to 45kWh.
Edit: found the video, related info starts at the 5:30 mark. It's a great interview if you haven't already seen it. Also this was early last year, not earlier this year like I previously mentioned.