If your going to Chicago the general plan is to keep the longs distance passenger in one coach. This way the conductor does not have to hunt for passenger. It also provides a quiet place for people to sleep. This of course does not work out perfectly, but it’s the plan.
Can you switch, not really. You can ask about relocating to a less full coach, but it’s a not likely unless your in a coach designated for shorts. Your conductor stuff people in cars by final destination, and the attendant does not normally have a manifest available.
On the Lake Shore limited on a Sunday going west. You find the coach are pretty fill with college students. The pack coaches will empty out before Buffalo, and it will be quite empty going across Ohio in the morning. The LSL is the last train across NY, so it’s quite busy.