I'm looking to purchase 3 economy tickets for an upcoming US>Brazil ticket and get a cabin upgrade after the purchase. I have the standard AA and an UA credit cards and after COVID no elite status anywhere.

United offers the possibility (of course, pending availability) to purchase an upgrade for all ticket classes besides basic economy (N).

a) United and American require a booking code above the basic economy to be eligible to purchase cabin upgrades. This is probably the case with most other carriers.
b) Do some carriers require that you are an elite member to be eligible to purchase? Or you that you pay with miles?
c) Is it safe to assume that this price decreases the closer you get to departure date, as long as there is availability?

TIA