I'm in the same situation with a pre-order for an R1T (with Max pack) and wanting to switch to the R1S (also with a Max pack). I contacted Rivian about Max pack availability for the SUV and was told that there would be an announcement about that early in 2022. I did not ask about what the model change would do to my place in line but it almost certainly will mean being pushed to the right.
My guess is that no one will even be able to configure an R1S with a Max pack until late 2022, with delivery perhaps starting in early 2023. Since my pre-order has only been in for about a year, I would also guess that the combination of a place pretty far back in line and the delays in R1S Max pack production will translate to late 2023 or early 2024 delivery for me.